Look at dri.sourceforge.net for docs on DRI. 3D is set up for radeons when you setup it for xfree86 most of the time. Try "glxinfo" if it mentions DRI it is good.
It is useless to switch to a voodoo3 now. Voodoo3 cards use DRI too so setting up one is the same. (except the driver name is different tdfx instead of radeon)