It's not OK to say to a potential employer, "You are my 3rd choice and I want to wait to hear from my top 2 choices before deciding whether or not to take this offer." Each potential employer should feel as they are your 1st choice. So schedule your interviews carefully. Schedule your interviews with your top choice first ... and work your way down from there. Try to delay your "lower choices" until after you have had a chance to hear back from your first choice. That can get tricky, but you can get into some very awkward situations if you don't do that.
It is OK to say something like, "I need a couple of days to think this over. I make it a rule to at least sleep on decisions as important as this one. I want to digest what you've had to say today and talk over the details with my family before making a committment." That can delay things a few days, but not forever. That's why you shouldn't schedule you "lower choices" to interview with first.