I can understand your dilemma. This is something I guess only you can take a real call on. There are way too many factors, the probability of your acceptance in the other two applications and your confidence to reject this.
If you aren't so sure, accept this offer and then maybe transfer next semester?
If you are asking for an extension, I would ask the professor I am in contact with personally on how to approach the school. Other than that if they gave u an aid means they consider you valuable so asking for an extension isn't going to hamper your chances. Try an ask, maybe oyu might see their true colors on how much they want you in their university