Don't use the 3dfx OpenGL renderer. Don't rename or copy 3dfxgl.dll to another file. Just use Default OpenGL.

3dfxgl is intended to be used with Voodoo1/2. Voodoo3 has a full OpenGL ICD so using "3dfx OpenGL" is not necessary.

Quake2 doesn't require DirectX 7 (or DirectX at all - it can use GDI and Waveout), though your V3 3dfx driver will require at least 6.

Great, thank you! The only other 3dfx product I have is a Banshee, so I've been used to using the 3dfxgl after renaming it.

Is the miniGL worth using over the default OpenGL?

No. The MiniGL existed because 3dfx did not release a full ICD at the time. Voodoo 1 / 2 being secondary video adapters that hijack the VGA signal for it to be used wouldn't work well with a general purpose full GL ICD. Later on around Spring 1999, 3dfx started including a full ICD for use with Quake III Arena in their v1/v2 drivers. That's also different since it still requires explicit support and still isn't for general purpose use. Before that, for V1/V2 it's MiniGL or nothing (or if you're clever to find the betas of the full ICD with some games...)

By the way, the same "don't use MiniGL" advice also applies to the Banshee.