Hmmm...

I believe there is nothing in a normal Voodoo3 Driver that allows to capture a Video Stream.

The only 3dfx Application capable of doing this is the VisualReality I think, although I only ever used it to capture the TV of the integrated tuner of the Voodoo3-3500/TV...

There are 3rd party programs existing (Soundforge, Open Source), although I don't know if these are TV related, or allow to capture any stream (e.g. Overlay Surface)