It depends on whether you are rendering to the same format as your source and if you process all of your video frames or not. If most of your video has no processing then Vegas will smart-render AVI and MPG source which is just a file copy. At that point having three drives really speeds things up considerably.
But I agree with Danny, if you are color correcting or adding other FX that require processing time, (which for many of us is most of the time) or rendering to a different format (e.g, AVI to MPG, M2T to MPG, etc.) then your render will be CPU bound and disc I/O plays a very small part.
The easy way to tell is to look at the disc activity light when you render. If it is just blinking on and off, then disk I/O is not your problem. If it is solid ON, then a 3rd drive will help.
~jr
https://www.johnrofrano.com/