a) It was held every 5 years, not 3
b) It hadn't been held for a couple of hundred years

And I don't see why it doesn't make sense. I don't remember entirely, but did Molly actually say "you need dress robes because there will be a ball" or "you'll know why soon..." Because we know that she knew ahead of time that the tournament was going to take place, the kids didn't. Maybe it was expected all students would obtain them once at school. Or already own them.