Student or Learner
Do these three words have all the similar meanings to mean a dull joke? I suspect "flat", while the other two I have heard a lot.
ex)The joke is so corny(flat, lame)
A corny joke is a clichéd or unsubtle one, so it's not necessarily the same as a dull one, but it doesn't work either.
Corny jokes can be pretty funny. You're just usually a little embarrassed that you laughed. The recent post about not taking a dog to a flea market is kind of a corny joke. They are usually safe to tell small children.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.