There is a way to train three master builders at the same time, I've done it on all four of my villages. As long as they're already at adept, you should have no problem making them all master builders at the same time after you get the cutting tool. I think that actually is the way that you are supposded to get them. It's the only way I have figured out that doesn't involve training one at a time, which risks one of them dying before you get the others there.