Who are the three who'll retire? I can think of Stevens off the top of my head, at 88 he'll either die or retire anytime now, but I'm not sure who the other two are. If they're Souter or Ginsburg that shouldn't change the balance of power, you'd have to search pretty hard to find a Justice more liberal than Stevens. Replacing Kennedy with a liberal would change the balance, as Kennedy tends to be the middle Justice who is usually the 5th vote in a 5-4 decision. If the replacement is for Scalia or Thomas, needless to say that would change the balance a lot, but I don't know that either would retire during the term of a Democrat.
All just speculation, but again, I'm curious as to who the two that you're referencing are.