Small cell is an attractive and promising technology for improving capacity in traffic hotspots using cell densification. In this paper, we propose the capacity enhancement for small cells under macrocell-assisted architecture utilizing the flexible 3-dimensional (3D) beamforming facilitated by the adoption of the active antenna system (AAS) at base stations (BSs). In contrast to conventional macrocell network, more dynamic and flexible 3D beamforming with narrow beamwidth is feasible in the small cell layer because the service coverage and mobility robustness are basically supported in the macrocell layer. This dynamic beam adaptation in full dimensions can improve the received signal quality and at the same time control the interference more effectively, which is especially useful for the dense small cell deployment scenario where the interference issue is one of the major concerns. In particular, performance comparison between the conventional sectorization with fixed down-tilt scheme and UE (user equipment)-specific 3D beamforming is studied. Furthermore, a novel UE group-specific 3D beamforming is proposed as a more realistic operation compared to UE-specific beamforming. System level simulations demonstrate the significant gain of capacity enhancement with 3D beamforming over the conventional sectorization with fixed down-tilt in terms of both the cell average capacity (up to 124.8% gain) and the cell edge user throughput (up to 454.3% gain). It is also shown that UE group-specific beamforming can achieve performance comparable to that of UE-specific beamforming.