It will be hard to separate them without causing harm to the roots. Plant one set per pot or, choose two and cut the third off at the rootline. When you plant, read the label on the potting soil package. If it contains nutrients, don't add more.
Bury the tomato plants so just the top couple leaf sets stick out of the soil (yes, you will be burying some leaves but that's ok). Tomatoes root on their stems so burying the stems will make healthy, better rooted plants. You can bury them straight down into the pot or slightly on their sides (planting on their sides helps when pot mates are different heights).
Keep the soil evenly damp. The biggest problem with container grown tomatoes is Blossom End Rot. It is caused by a lack of calcium in the root zone when a tomato first forms. Its not caused by a lack of calcium in the soil but rather, the tomato plants can't utilize the calcium when the soil is too dry.