Three-dimensional (3D) wireless sensor networks (WSNs) have recently received attention due to their wide-range potential applications. Although some studies have been done on 3D WSNs design, the phase transition phenomena in 3D WSNs has not been adequately analyzed. In this paper, we focus on extending a threshold analysis for full area coverage in WSNs from two-dimensional to three-dimensional case using an analysis technique called bin-covering. We also conduct additional experiments to demonstrate the accuracy of our analysis. Our results can be helpful in understanding threshold properties of full area coverage in 3D WSNs.