Actually, k6, k6-2 and k6-3 have a good 3d performance if the software was made to use its 3d!Now instructions. As not many softwares does, its FPU performance is usually lower than a PII. However, AMD has surpassed Intel now that Athlon has a better FPU and can use an upgraded version of 3d!Now(also called 3dnow). PII can only use MMX that does not accelerate FPU instructions. PIII can use SSE that is Intel's answer to 3d!Now.
Anyway, both K6 and PII are old generation processors and that time intel was still in its wonder years. Nowadays, if you want a CPU and you are in doubt between an Athlon, a PIII and a PIV, amd's one is surely the best choice.