It's pretty common to have a separation. By the time you've had 3 pregnancies, I think something like 80-90% of women will have some degree of muscle splitting. Only consider surgery if you're done having kids! Pilates is supposed to be good, but no crunches - I discovered I had a separation on my own after DS, and just doing pilates pretty much closed it right up. However, it was only about 2-3 fingers to start with.
Anyways, there is a lady who has a blog with lots of great info on her battle with diastasis. The blog is raising arrows dot net, and you can search for the posts on diastasis recti. Or, if you can't get at it - ask me to PM it to you!