If using a molcajete, grind garlic and chiles to a paste. Grind part of tomato to a paste, or reserve all of it to fold in at the end. If you don't have a molcajete, chop garlic and chiles finely, then proceed using a bowl and fork.
Cut avocados in half and peel. Remove pit by inserting the blade of a knife into pit and twisting. Mash avocados into chile mixture, leaving some texture. Add salt to taste. Stir in cilantro and tomato, reserving a little of each for garnish. Serve immediately.