They are all foods that treat a broad spectrum of gram positive and gram negative bacterial issues in your fish. They are basically different kinds of antibiotics, and if one doesn't work for your fish, you can always switch to another.
Penicillin
For fin rot, cloudy/bulgy eyes, mouth sores, bacterial infections, and septicemia.
Tetracycline
For gill disease, bacterial tail rot and infections, ulcers, sores, dropsy, popeye, columnaris, aeromonas, pseudomonas and chondocuccus.
Ampicillin
For bacterial infections, popeye, swollen body, bacterial kidney disease, fin rot, flaxibacter infections and septecemia.