Revenge and blindly assuming they were wrong and you were right isn't a healthy thing for you to do in this situation.
It would be wise to think back over how long you have been friends with these three others, and what kinds of relationships everybody involved has with one another. You may also want to compare humors, personalities, and how educated these people are on social issues. You may very well have been insulting somebody without realizing it, or they insulting one another and taking it out on you. That will be a rather important skill for later interactions.
For this, however, I doubt there was any serious fault in any of the involved parties, especially if using the childish nature of their note as evidence for what they were experiencing.
Tl;Dr, don't get revenge... think a bit more about what happened before you draw conclusions, especially as you get older and people begin granting you benefits in education or career. PS your friends were rather immature about ending their relationships with you. Bleh.