You perhaps stand a pretty good chance of those 3 kids turning into 6 adults and maybe 6 to 10 small people.  Your 1500 sq ft will likely get small again before you know it.  We had 1288 sq ft originally, w/us and 2 boys. Now there's a dozen or so for holidays and it would be cozy if we hadn't added 12' x 24' when the oldest son was in college.