The K6-2 was based on the Nexgen 686. AMD bought Nexgen and used there RISC86 core which allowed much higher speeds then the old K5 core. The K6-2 is merely an enhanced K6. It included 3d Now! which is 21 addictional instructions for FPU intensive tasks.
It scaled in speed from 200MHz to over 550Mhz Extending the life of the socket 7 platform by several years. It was also made in mobile and embedded versions (lower voltage and higher temperature tolerance.). It also used busses from 66-100Mhz.
The K6-3 integrated 256k of L2 cache onto the CPU die in order to boost performance. Coupled with upto 2MB of L3 cache on the mother board the K6-3 almost NEVER had a cache miss and was a very good performer. unfortunately at 21.33 Million transistors they were expensive to make.
Starting with the 400 a new core was released the 'CXT' core was faster and more efficient in its caching routines.