You are in a tough situation. My advise would be - don't over do it, lower your expectations. Pick out a few must see/do things and be happy.
With a 3 year old you will need to take it easy. You will need to either shorten your day or take a break mid-day (which will be a little more difficult staying offsite).
Are you getting single park tickets, or parkhopper and just going to have a day(s) left over for your next trip ?
My advise - which is probably unpopular for this board - use this trip as a preview for a future trip.
I do not know the sleep requirements of the youngest, but my 3 year old still needs about 11 hours of sleep. Mine could make one real long day, but not two or three in a row.
Unless you plan on returning in the next 1 - 2 years, I would buy single day tickets, not mess with multi-day passes that you may loose sitting in a drawer at home. I would probably spend 2 days at MK and another day at either AK or MGM, whichever you feel the girls would most enjoy.
I would arrive early, and plan on spending 6 - 8 hours in the park dejour, then going back to the hotel, relax, get to bed early so the girls, especially the youngest is not over tired.
I agree with the above posts - go early to Fantasyland and Toontown. Although check the schedule, I'm not positive all the Toontown attractions are open immediately when the park opens.
CRT would be wonderful. As stated reservations are taken up to 90 days in advance, call on the 90th day (March 24?).
I'm sure some of my comments are not the norm, but if I was in your position, it is what I would do.
Have a great trip, just keep your expectations in line with your children's ages and your limited number of days.
Jeff