Sprint's network has been undergoing a major reconstruction in the last year or so, but there are still some problems to be ironed out. Suspiciously, it has been Sprint's policy this whole time to pretend that the super-slow 1xRTT network doesn't exist. Instead, its devices just display 3G like everything is okay. The new Nexus 5 is apparently playing ball with Sprint too – if you're in a 1x zone, the phone will still read 3G.
HTC has an above-average track record with software updates, but they appear to have misstepped with the most recent PRL version for the EVO 3D. For some unfathomable reason, said PRL (version number 50580) seems to be blocking Sprint's 3G network for a lot of users; as a result, they are left with no choice but to rely on the considerably slower 1xRTT technology (2G) for data.
Fortunately, there's still hope for those who unknowingly applied PRL 50580 - simply revert to the previous PRL (21081) using the instructions at Good and EVO.