It's not too surprising to hear of the desire of the admissions committee to want a commitment quickly at this late date. We are at May 1 now, and time is ticking away.
Part of the reason for the request for a relatively quickly commitment could be that they want to roll your spot to someone on the waiting list if you decline your spot.
The advice here about contacting the school and requesting more time is solid. Be professional, conduct yourself with tact and diplomacy in this request.
The other action you should take is contacting your other two schools and let them know you have an offer from another school with a deadline of X. Ask them if there is any way they can give you the admission decision from their school before X date so that you can consider all of your offers at the same time.
When you contact the school to which you are admitted, you can also let them know that you've asked the other schools to give you a decision more quickly.
In the end, you may have to pay the deposit in order to hold your space. However, there's quite a bit you can do to influence the actions of the respective admissions committees before you get to that point.
Good luck!