A variant of the high five, a low three is when two homosexual guys touch their genitals together, naked or not. Can also refer to when two non-homosexuals bump into each other and their penises touch.
Paul: Max and Chuckie are so gay for each other, they slapped a low three in public the other day.
Justin: Oh yeah, yesterday I bumped into Eric in the hallway and we had a low three. It was so awkward.