Back in the days when I was a kid, I bought a used Voodoo 2 3dfx card and installed it in a PC running windows 95 at the time. I installed the driver from the CD and used the passthrough cable that came with it to pass the signal from the 2D GPU through to the voodoo.
However I never got it working. Games I tried were half-life 1, CS 1.5 (before the steam days) as well as some other games. But none of them would run in 3D accelerated mode, only software rendering. Selecting Direct3D in half-life did "work" but I assume it was still just CPU rendering for compatibility.
Maybe I wasn't as tech savvy back then and I cannot remember what I did trying to troubleshoot it. I considered that maybe the problem was that I had not selected 3dfx mode in the games' settings. However reading up on the subject now, there are places stating that the cards would also work with OpenGL and Direct3D "whenever those APIs are called for". Which means that even if I failed to select the appropriate mode, it should still have run with 3D acceleration in those modes. The issue could have been anything, from a broken windows setup to bad drivers.
The reason I bring it up now is that I found the card again and have for some time wanted to make a retro DOS/win9x setup for the nostalgy (even though I know there are emulators for modern PCs etc). And I want to see if I can make use of that card