I am in a love square.
What do you mean by a love square?
I'm in love with my boyfriend, his best friend, and his best friend's fiancee
Love is a great thing, but you have a boyfriend already. You are in a relationship only committed to him, and you should be fair to that. It is emotional cheating to have feelings for other people but him and you may be betraying him.
I'm loyal to him and I don't understand how that is cheating.
It depends on your boyfriend and his thoughts. Some people do not see an emotional connection as cheating, and some do. To play it safe, you should refrain from acting on those feelings.