Heat a medium skillet or comal over low heat. Place the tomatoes, jalapeño, and garlic on the skillet and toast them until brown spots develop on the garlic but it does not become black, about 3 minutes. Remove the garlic from the pan.
Continue roasting until the jalapeño and tomato skins blacken in places, the flesh begins to soften under the pressure of tongs, and the skin from the tomatoes starts to come away from the flesh (the flesh will soften and begin to release its their juices), about 20 minutes. If one ingredient starts to blacken before the others, remove it from the pan as needed.
Heat a separate small skillet over low heat, add the chiles de árbol, and toast for 1 to 2 minutes, turning often, until they darken in color on both sides, taking care not to let them burn. The seeds will come out as you toast the chiles—you can use them or not depending on your heat preference.
To prepare the salsa, begin by adding the garlic to a molcajete (mortar and pestle) and mash until smooth. Add the chiles de árbol and break them up until a coarse red paste forms. Add the jalapeño and continue to mash until the mixture is fairly smooth but with some pieces remaining. You’re looking for a rustic, chunky salsa, but make sure to break it up enough so no one gets an overly large piece of jalapeño.
Last, add the tomatoes and mash them in carefully (they will be filled with hot steam) until just a little chunky but with no large chunks remain. Season with salt and use a large spoon to mix and scrape down the sides of the molcajete. Serve the salsa in the molcajete or a decorative serving dish.