You don't have to worry about voltage drop, 75 feet isn't far. You need #3copper, or #2 aluminum. If you can find it, you can use 2-2-4 aluminum, which is the three conductors you need already bundled together. If you use aluminum, which I would recommend, cuz copper is insanely expensive, you need to lightly coat the exposed wire with an anti-oxidizing compound. You can get small tubes of it at the local big box stores.
Is this an attatched garage or detatched? If it is detatched, you need to drive two ground rods and use a #6 ground wire. If attatched, just run the ground wire from your service panel to the subfeed in the garage with the other conductors. Hope this helps.