If Blu-ray drive for PC supports 3D, it means its minimum reading speed is 4X.
Even though its maximum speed is 6X, some drives do not support 4X on inner disc section where the drive reads at slowest speed.
By keeping the minimum reading speed over 4X, the Blu-ray drive can enable the CPU realize 3D on 3D monitors. (CPU decodes: this BD does not have GPU nor CPU inside.)
In addition, USB 2.0 is fast enough to realize 3D on PC as the data is still encoded while transferred from BD drive to PC.
From PC to monitor, the connection should be HDMI, though.