This study aimed to compare the marginal and internal adaptation of two bulk-fill and one conventional composite resin placed with different techniques using micro-CT. Twenty-one extracted human third molars with standardized mesio-occlusal cavities (a bucco-lingual width of 3 mm, a mesio-distal width of 5 mm, and a depth of 4 mm) were randomly divided into three groups (n = 7): Group 1 was restored with SonicFill 2 using sonic activation and 4 mm single layer, Group 2 with Tetric N-Ceram Bulk Fill in a 4 mm single layer and Group 3 with Clearfil Majesty Posterior using a 2 mm incremental technique. All restorations underwent 5,000 thermal cycles before micro-CT scanning. Data were analyzed using Kruskal-Wallis and Mann-Whitney U tests (p < 0.05). The SonicFill 2 group showed a significantly greater marginal gap volume than the Tetric N-Ceram Bulk Fill group (p = 0.023), while no significant differences were found among the Clearfil Majesty Posterior group and the others (p > 0.05). No significant differences were observed among the groups in terms of internal void volume (p = 0.101). These findings suggest that variations in marginal adaptation among different composite resins may be attributed to differences in their material composition and placement techniques which have a limited effect on internal adaptation.