I don't think three days is too much, per se, but I think you will find the kids LOVE epcot and that you'll be disappointed if all you have is a half day there. At toddler age, I actually found epcot better than MK.
I also think at least one night you should keep the kids out for one of the nighttime shows if you can swing it (Illuminations or Wishes, either one)
I also think at least one night you should keep the kids out for one of the nighttime shows if you can swing it (Illuminations or Wishes, either one)