Wear disposable latex gloves before cutting any peppers. Pepper residue can remain on the skin despite thorough hand washing and can burn eyes along with other delicate areas.
Prepare 6-8 1/2 pint jars with lids by cleaning them in the dishwasher or hot soapy water.
Combine 3 cups of water, vinegar, sugar, salt, and garlic in a large saucepan. Bring mixture to a boil over high heat. Stir in the jalapeño slices and cook until they turn from bright green to a slightly duller green, about 7 minutes.
Use a slotted spoon to transfer the jalapeños, garlic, and seeds to the jars, dividing them equally among the jars. Spoon the liquid into each jar, filling each to 1/2" below the rim.
Close jars tightly. Let cool completely, then store in the refrigerator for up to 2 weeks. You can use them immediately, but they will get spicier the longer they sit.