Research and development of heterogeneous small cell in cellular network accommodates the proliferation of data-hungry devices and applications. Meanwhile, for challenges in providing high-data-rate transmission in poor coverage area, utilizing unmanned aerial vehicles (UAVs) provides a promising solution attracting tremendous attention. However, tight integration of UAVs creates an obstruction in existing network to acquire high efficient resource utilization, which needs investigation in 3D network architecture. In this letter, we propose a resource allocation optimization mechanism to minimize mean packet transmission delay in 3D cellular network with multi-layer UAVs. Numerical results demonstrate effectiveness of the proposed algorithm, where optimal spectrum and power allocation can provide minimum packet transmission delay.