M.R.
Our daughter (now 14) has done Mathnasium each summer since the end of second grade, so that's six summers now. We only do summers and she does not go during the school year. She goes just to keep up skills and be ready for the fall. My husband's undergrad major was mathematics and he always says that "Math is a muscle and has to be kept exercised." It's true-- practice does keep math skills strong but it doesn't have to be tons and tons of practice, just the right, focused practice.
Mathnasium could indeed be expensive if you send three kids, as the first poster below noted. I would advise you to go visit the new Mathnasium near you and find out what they would offer your one child who needs the most help so he can be fully mainstreamed by this fall.
Kumon sends home piles of worksheets, I'm told. Mathnasium will not (unless somehow your site is very different from ours, and I doubt it). In fact they consider their worksheets to be proprietary so they don't let them out the door or ever send them home. The work is done there in the facility and there is no homework.
A kid goes in; gets his or her binder that's always kept there and has the exercises already in it based on what you and the director want the child to achieve; and sits down. These are not exclusive one-on-one sessions where it's just your child and a tutor alone together the entire time. Your child will be in a room with other kids, and the tutors will sit with your child, explain whatever new concept (or reviewed concept) your child is doing that day, and leave your kid to do it. Then the tutor moves on to help another kid who's there on whatever that kid is doing, and once that child is getting to work doing exercises, the tutor will swing back over to your child to see how things are going, check exercises if they're finished and then show your child what was wrong or right etc. It is a better use of time, to me, than paying a tutor just to sit there staring while your kid does exercises. We've always found that the tutors were good at explaining things. All of them at our location are usually college undergrads or graduate students. For younger kids there are prizes for completing a certain amount of work.
Since your Mathnasium center is new, ask them about who their tutors are, what level of experience the tutors have, how flexible they are about hours (ours asks you to set up times each week to come, but frankly, they are very relaxed about it and if you don't turn up and just come the next day instead it's fine--just get in all the hours for which you pay). The center director should work with you to tailor the binder's contents to what your son really needs. Spend time being sure that the center understands he goes to special ed and might need different or additional attention and ask if they are prepared for that -- If you feel they aren't, don't go.
Mathnasium works well for our daughter. Memorizing times tables etc. is also something very good to do with your kids, as is getting them some of the fun tools available to help kids remember multiplication and division facts (ask their teacher this year about that for summer study). Ask a lot of questions and if it doesn't seem right for your family, look into experienced private tutors. I would not go with Kumon due to the expectations of lots of worksheets they have to do at home. Be sure your other two kids do get practice in the summer, even if they don't need tutoring or Mathnasium.