I didn't had any problems with Jack or plugins - the problem with 3.x was Midi editing. Well it was introduced in this release, but I did some drum programming (our drummer was sick, so we've programmed drums to our music) and it sucked.

It was totally normal that Ardour crashed at least twice in the 1 or 2 hours when we were editing drums. And the devs knew that this problem existed, but they've basically told, that it should be fixed in 4.x.