I'm brand new to gardening myself, so I would suggest you wait for others to chime in as well, but I wanted to add that my snap peas aren't very big at this point for me either in Zone 4. We had pretty chilly weather until late in the spring, so I think that may account for some of the slow start. I also was under the impression that snap peas could be planted pretty densely without issue.so I wouldn't think three would be a problem (but again--I'm no expert, so please wait for others to comment!).

I'm growing Sugar Anne Snap Peas now but want to try Oregon next year. Good luck!

3 plants per square foot is far from overcrowded. In fact, it's underplanted. Can't help with the size of the plants but they more than likely have started setting pods by now (you've probably even harvested already). And I'm sure you've noticed by now that you need a lot of plants if you want a decent sized harvest. That's one downside to peas.

Rodney