The K3 pliers are the newer version of the K2. Both pliers are still considered the "basic" pliers to install plastic snaps. They work identically.
The only difference is that the tip of the top shank on the K3 easily screws off. With the K2, the entire shank had to be removed by dismantling the pliers and then putting everything back together again.
So the difference is simply convenience should you ever need to change the top shank (which most people never do).
Neither pliers are multi-functional. They will not work for anything other than plastic snaps.