A 3 Game Guarantee Tournament simply means that every team is guaranteed to play a minimum of 3 games. A 3 Game Guarantee Tournament is set up similar to double elimination tournaments. There is a winner's bracket and a loser's bracket, the main difference being teams that lose their first two games will get to play an extra game in the loser's bracket in order to assure everyone gets to play at least 3 games.

A team that has already won once in the winners bracket is already guaranteed 3 games as they will be playing their second game in the winners bracket and then dropping down to the loser's bracket if that game is a loss.
A team that loses their first game and wins their second game(losers bracket), will already be playing 3 games.

The championship game works the same as a double elimination. In the championship game, the team coming out of the loser's bracket must beat the team coming out of the Winner's Bracket twice.