Sounds like they are about done, but you will know for sure when the tops go down and dry off on their own, or if you are impatient you can knock them over and then wait for them to dry down. Don't expect Sweet Spanish or Candy onion size from Copra, but 3" is a very decent return for a cooking onion, and probably they aren't all going to be that big anyway. Best to wait them out, but it shouldn't be very long now. The outer skins starting to color up is the signal of that.

If you planted copra plants in april in the PNW, they shouldn't be finishing up yet, or at least thats what I think based on my experience growing here near portland for 20 years. I'd expect them to go another month before the tops start falling. But, if they are water or nutrient stressed, they could start early.

A photo would help if you have the chance. Here's one of some of my spring planted onions, the variety is Cortland, similar to copra but a little bigger. Notice that there's very little bulbing yet.