Wait a minute, do we go on three, or after three? Playing Rhythm Heaven Fever, I feel a bit like Elizabeth Berkley in that scene in Showgirls where she first joins the show..
When you say downbeats, I think you mean off-beats Tom and Mercanis. Downbeats are the first beat of a bar and everyone (within reason) should be able to count to do them. Off-beats, however, are the 2nd and 4th beat of a standard 4-beat bar. If you have trouble, use both feet to count the beats. Left - right - left - right.
Then hit whatever you want when you tap your right foot. Now you're playing a syncopated rhythm!