I would use goofing around and messing around interchangeable. However fooling around has another meaning ie. fooling around with sb. Kinda means when two people, usually not currently in a relationship, find themselves kissing... and probably more... hahaha. for example: A: Is Mark your bf? B: Nah, he isnt, we are just fooling around.

By themselves they are all the same. It all depends on the context. English is a very rich language and therefore ambiguity arises. Also the words be innuendos and euphemisms. In computer science, context free languages are those that can be processed and "understood" by computers. Non context free languages such as natural languages cannot be understood by computers of today since they do not posses enough memory to store all contextual information. That's why mechanical translators are useless except for narrow fields such as law and technical subjects where meanings are precise and unambiguous. Consider the following sentence, without context it is ambiguous, "The Queen threw a ball".

I would use all three interchangeably, though as Jess points out, fool around, does another meaning, as does mess around to further add to her explanation about fool around, yes we use it when people not in a relationship partake in flirtatious or sexual behaviour, we also use this and messing around, when we talk about people that are being unfaithful to their partners. Everyone knows his wife is fooling around/messing around behind his back with the office manager.