3D plane-wave imaging systems can support the high volume acquisition rates that are essential for 3D vector flow imaging and sonoelastography but suffer from low resolution and low SNR. Coherent compounding is a technique to improve the image quality of plane-wave systems at the expense of significant increase in beamforming computational complexity. In this paper, we propose a new separable beamforming method for 3D plane-wave imaging with coherent compounding that has computational complexity comparable to that of a non-separable non-compounding baseline system. The new method with 9-fire-angle compounding helps improve average CNR from 1.6 to 2.2 and achieve a SNR increase of 9.0 dB compared to the baseline system. We also propose several enhancements to our beamforming accelerator, Sonic Millip3De, including additional SRAM arrays, configurable interconnect, and embedded DRAM. Overall, our system is capable of generating high resolution images at 1000 volumes per second.