There are a small amount of loops included with GarageBand that are 3/4 time. If you look at a list of loops, there is a column with the number of beats in that loop. Look for 6 or 12. You'll have to set GarageBand for 3/4 time as well.
Of course, you could "roll your own" using the built-in drum kits. If you have a pretty good feel for rhythm, you probably won't find it difficult. Once they're created, Software Instrument note loops work more or less the same way as audio loops.