Same thing is happening to me.  I got 6 copies of the K-3s for each K-1 recipient.  When you print the tax return as a whole, it is fine and only has one copy.  When I select the "Selected Forms > K-1's only" is when it prints out the multiple K-3s. 

It is an issue that Proseries needs to fix.

I didn't notice until I was a few hundred pages in and thought "Why is it still printing, I know the K-3s are extra pages, but it should be THAT much!"