That depends on how cold your garage will be. 5'x3'... so you're looking at 240W or so of light? On a particularly cold night in late fall /early winter, before you start your plants, try putting out 240W of anything where your plants will be in the evening, then take a temperature measurement under it in the morning. If it stays above 55F, there's not much that will die, and the less cold-sensitive plants (such as lettuce) should thrive. Above 70F and everything should thrive.

If you want to raise temperatures, you could always insulate your "greenhouse deal". Use some of that aluminized bubble wrap or foam and you'll do double duty, giving your plants extra light (from the reflections) as well as trapping in extra heat. Just be careful not to overheat them!