Curious as to why you changed only 3 plugs?
If you put in heavier springs, then the centrifugal advance will come in later, which is not what your after. As cdminter said, your want all of the advance in by 3000.
In terms of the timing, IMO, I'd change the remaining plugs and clean off the 3 you already changed, then unhook the vacuum advance and plug the line. Set the total timing between 34 & 36 and find out exactly what rpm it is all in by, and then drive a WOT throttle session and recheck the plugs immediately and see what they look like.
It will not hurt the motor to run it with no vacuum advance. And, oh yes, MAKE sure your balancer is marked accurately for #1 TDC.
I didn't see where you checked you vacuum, where is it at idle? Did you check for leaks?