Site 3 at Koźliki (AZP 42-88/8) is located on the western edge of the village, on a high escarpment of the Narew River (Figs. 1 & 2). It features four earthen mounds, spaced over a distance of approx. 75 m. Two of them are round, with diameters of approx. 12 and 15 m (Nos. 2 & 3), while another (No. 1), measuring approx. 12×6 m, is oval in shape, which is the result of its destruction during the construction of the fence around the nearby Orthodox church. The worst-preserved mound, no. 4, is in the form of an irregular earthen embankment. All the mounds measure no more than 1 m in height.

In archaeological literature and conservation documentation, the site used to be described as either a confirmed or hypothesised early medieval barrow cemetery. Such an interpretation of the function and chronology of the mounds was based on ambiguous information obtained as a result of the exploration of one of the mounds in the 1860s and the excavations conducted in the early 20th century. The above-mentioned hypothesis was disproved during the excavations conducted in 1996 and 2014, which explored two trenches established in different parts of the site (Fig. 3). Trench 1, located near the river escarpment, yielded approx. 29 inhumation pit graves from the early modern period (Fig. 8). Most of the bodies were positioned with their heads facing southwest and placed in wooden boxes, whose bottoms or lids have rarely been preserved. Two graves contained iron nails, and three graves held fragments of early modern vessels. Bones were discovered in 22 grave pits. Most of them belonged to non-adult individuals (14 burials), including nine small children. Of the eight adults, it was possible to determine the sex of seven individuals—three women and four men.

Moreover, trench 1 yielded two likely prehistoric pits (features 1 and 72), three pits from the older phase of the Early Middle Ages (features 13, 29 and 38) and five pits from the Early or Late Modern Period (features 3, 32, 36, 64, and 70).

Trench 2 was opened in the southern part of mound No. 1. The upper part of the mound was composed of contemporary topsoil (layer 105) and dark grey sand (layer 106) deposited probably during the construction of the church fence (Fig. 24:2.3). The original mound was uncovered underneath these two layers; it consisted of approx. 0.8-m-thick layers of brown sand (layers 114/1, 114/2, and 114/3) and had three early modern pit graves dug into it (features 117, 121 and 123; Fig. 24). The mound was erected on sandy virgin soil, after the area had been cleared of the original topsoil, which has been preserved only on the edge (layer 108). In addition, the trench yielded two pits dating to the Late Pre-Roman Period or the older phase of the Early Middle Ages (features 131 and 133) and four early modern postholes (features 110, 119, 127, and 129).

The earliest stage of settlement at the site may be associated with the Late Mesolithic, as indicated by the flint material bearing techno-stylistic characteristics of the Janisławice Culture. This potential settlement episode is particularly evident in the structure and morphology of the blade blanks, as well as the presence in the assemblage of, among others, a trapeze (Fig. 13:11) and a Wieliszew-type point (Fig. 13:14). However, the occurrence of the flint artefacts with Janisławice Culture characteristics does not necessarily confirm that the site was used as early as the Mesolithic, as the items in question were found together with pottery (Fig. 16), suggesting several stages of the occupation of the site by Neolithic hunter-gatherer communities. Such a possibility is confirmed not only by the characteristics of the pottery, which includes fragments of vessels reminiscent of the ‘classic’ and ‘late’ styles of the Neman Culture groups, but also by the presence of a few flint artefacts of clearly sub-Neolithic provenance, such as a Sośnia-type projectile point (Fig. 13:12), an asymmetric projectile point (Fig. 13:13) and a flake side scraper-knife shaped with trough retouching (Fig. 14:6). It is, therefore, possible that the flint artefacts of apparently older provenance were part of the inventory of later sub-Neolithic groups. The cultural attribution of the few potsherds that are reminiscent of the pottery traditions of the Corded Ware and Globular Amphora Cultures is likewise unclear (Fig. 16:6), as similar material is also characteristic of later cultures, where it is defined as the Linin type.

The next phase of the use of the site corresponds to the Iron Age, probably the Late Pre-Roman Period. A clay spindle whorl (Fig. 25:1) and approx. 600 potsherds (Fig. 25:2–8) likely date to that period. Most potsherds are fragments of the so-called kitchenware, with external surfaces usually showing traces of light polishing, less frequently smearing with the so-called twig-bundle. The pot rims are most often sharply (diagonally or horizontally) bevelled (Fig. 25:3) or rounded (Fig. 25:7:8), sometimes thickened (Fig. 25:4:5). The tableware fragments, with polished surfaces and fired most often in a reduction atmosphere, are much less numerous. Among them is a fragment of a rim with a faceted edge (Fig. 25:6). In terms of production technique, the artefacts discussed are reminiscent of the pottery from the settlement at Zajączki, Białystok County, site 1, located approx. 10 km west of Koźliki. Unlike the site at Zajączki, however, the Koźliki site yielded no vessels with a brushed surface, which excludes its connection to the cultures of the forest zone of Eastern Europe. Based on the shape of the edge of one of the rims (cf. Fig. 25:6), at least part of the described assemblage can be associated with the cultures influenced by the La Tène circle. The form in question is reminiscent of the style of Przeworsk Culture pottery of the Late Pre-Roman Period, phase A3. However, determining cultural attribution and dating of the entire material on the basis of a single potsherd can be misleading; therefore, at the current stage of site exploration, the chronology proposed for the group of vessels under discussion should be approached with a high level of caution. The material from the older phase of the Early Middle Ages consists of 248 fragments of hand-made vessels (Figs. 20, 23, 25:9.10). Egg- or slightly S-shaped pots are the most common (Figs. 20:2.6.7, 23:4), while other vessel shapes are rare (Fig. 20:2; 23:1). The rims are most often cut horizontally (Fig. 23:3.4) or obliquely (Figs. 20:4.5, 25:9.10), less frequently rounded (Figs. 20:2.6.7, 23:2). The vessels described are reminiscent of Prague-Culture-type pottery. The material attributed to said culture is dated to the 4th/5th–7th centuries in the area of present-day Belarus and to the 5th/6th–7th centuries in the area of present-day southeastern Poland.

The prehistoric and early medieval settlement structures were severely damaged when a cemetery was established at the site in the Early Modern Period. The cemetery was located on the western edge of the village of Koźliki, a few dozen meters from the last buildings, and was likely established at the same time as the village, i.e., in the 16th century. It presumably ceased to be used in the late 18th century or early 19th century.

The dating and function of the earth mounds found at the site discussed remain controversial. The findings from the 2014 excavations definitively ruled out the early medieval chronology of the burials discovered in the mounds at the beginning of the 20th century and identified them as early modern graves, dug into the older mounds. However, there is still insufficient evidence to determine the time of the construction of the mounds, as they may have been erected either in prehistory (during the Late Roman Period or later) or in the Early Middle Ages. The mounds most likely served a sepulchral function; however, since no remains of original burials have been found in any of them, it is also possible they had a different purpose.