I agree with some of the previous recommendations, technically a lot of people would say they need a 10g minimum since they are so active. However, I think 3 MALES would be fine in a 5.5 so long as you don't slack off on water changes. Live plants would make the Guppies feel safer, if one started to harass another they would have a place to seek refuge. Plants would also help to keep your water cleaner since Guppies have a hefty bioload for their size.
Note that I say only males as you wouldn't have to worry about fry and the possibility of a couple growing to adulthood. I've always kept males only and never had a problem aside from their showing off to each other.
><(((º> Aaron ><(((º>