Indeed, we needed a plum forum. I live in zone 4b, quite sandy soil, and I tried a few but always been disappointed: they died or didn't fruit and most of them are cling stone type. On top of that, we live on a flat
land, in very continental climate, so the temperatures vary wildly. We just has a serious cold snap with 2 weeks of -20-24F below zero and in 5 days, we have +43 today but another cold snap is coming, so....
We are also very prone to late frost, of
course, so I'm riveted by your attempts and took down the list. If they survive in zone 3, they
should work well here, if I can even get them. Would you have some that are free-stone?
Mount Royal, Pipestone, Toka, Waneta and Black ice is pretty much what we have to pick from. Mount Royal is a European type and seems to work the best, but I got very few fruit before it died. Toka and pipestone also worked for 2 years, then they became infested by something that ate the leaves [tent caterpillars] and they are failing now.
Let's get a great forum going with lots of different kind: Several are bound to survive and perform!