HoosierMommy, you may find that as DD-almost-1 gets closer to 15 or 18mo, her and DD3 will be much more likely to play together. I have 3, all ~2 years apart (DS7, DS5, DD2.5) and 15-18mo was the magical age when the younger one was old enough to participate in games with the older kid(s). My boys loved making block towers for DD to knock down or telling DD about their trains and letting her try to run them on tracks they had made. I really think it just helped that there was someone there to watch the older kid play, especially a younger sibling who thought the big kid was so cool, lol. I think their toys also get a second life at that time because they can show off all the neat things they can play with.
