Odd Triplet

I'm trying to write a measure beginning in a triplet. I tried adding notes and always ended up with only two of them being in the triplet. When I hit undo, I get a triplet bracket thing, with a quarter rest, an eighth rest and some of another quarter rest under it...I tried to use a whole rest and start over, but it won't let me put anything in that measure, and skips over the whole thing. After a couple of tries, the program just gives up and stops responding. Most annoying.