Just listed with a realtor for 3 months but decided I would like to change to another. Is that allowed or legal?
Since you have a contract for 90 days, you would have to get your agent's permission to be released from the contract so you can go with the other agent.
Most of the time they will attempt to dissuade you from making the change, but will release you because they don't want to engender any bad will. The agent will be reluctant of course, especially since you just "changed your mind," and cannot point to any lack of performance problems with your present agent.
The agent can refuse to release you from the listing agreement and there are various reasons they may do so. In that case, you're either stuck or you have to simply take your home off the market for the rest of the listing period.