Both of these were suggested to you already, but if you aren't already familiar with them, I'd like to tell you more! My children preferred co-ed parties until they were in elementary school, so we usually ended up alternating between Gymboree Play & Music and The Little Gym for their parties. For a very reasonable rate you get the entire gym and staff for your party, with the activities customized for your children. I can't say enough about how fantastic these parties can be! My husband got "bored" with doing a gym party for every birthday as the years went by, so we began to try other things, and he hasn't liked any of the other parties as well (the kids, of course, have a fantastic time anytime they are the center of attention and are surrounded by friends, but also have the best memories from their Gymboree and TLG parties). As a parent, it doesn't get any easier or more fun! The Little Gym and Gymboree provide everything but the cake and plates/napkins/plastic silverware/cups, which allows you plenty of freedom with the party theme (I love the idea of letting each child pick plates/napkins/etc), while keeping it simple. The day of the party, you just show up with the birthday boy and girl, cake, plates, etc, and any supplemental party favors you might want (there will be favors provided) about 15 minutes before the party starts, and then the children are completely engrossed in the fun (and have a chance to use all that manic three-year-old-birthday-party energy in a productive, fun and supervised way). You are free to actually VISIT with other parents (how often do you get to do that?!) and/or film/photograph the party.
As to which I prefer (Gymboree Play & Music or The Little Gym), I chose based on where we lived/which was closer to more of the invitees and/or which one the birthday child was then taking classes. I had fantastic parties at the Gymboree in Plano, and the Little Gym in Dallas (Preston-Forest), Richardson, and Allen/McKinney (TLG Allen moved to McKinney...same staff; I've had parties at both locations). I've never hosted or been to a party at either where the children didn't ALL have a wonderful time.
Good luck to you! I'm sure your children will have a fantastic 3rd birthday!