I've got 3 finches, and they got along together until....?

Its not uncommon for finches to pluck other finches. Its usually a female plucking a male who usually just sit there and take it. Its especially common with Zebra finches. There's not much you can really do, I've watched this with a fair number of pairs I've had over the years. They still seem quite happy, despite being somewhat bald and funny looking. Unless they plucked guy is really objecting, I wouldn't worry about it.