T3 is a thyroid medication. Generally if you have a thyroid problem, you may be prescribed one of the T4 products. Normally your thyroid regulates your metabolism. T3 will "enhance" your metabolism - however not only does it "eat" fat, it will also eat muscle. Competitive (& non-competitive) body builders often use T3 in conjunction w/ an AAS to help keep body fat down. The AAS helps to counter the muscle loss that also is caused by T3.
T3 is not an OTC drug & you can't get it in a store. Some of the commercial brands are Cytomel & in Mexico it is Cynomel. It is not a steroid.