You can definitely fit in more than three cities unless you want to move really slowly, and you might have your reasons for doing so. If you decide to do a multi-country route, which I think is fine over two weeks, do an open-jaw flight from New York City or Boston and pick two cities where it will be easy to travel back to the United States, given the aforementioned European flight problems. If you can get back to NYC or Boston from cities like Paris, Rome, Budapest, and Prague, then you can connect domestically from there. That will minimize your connections in Europe itself. Some good "A to B" open-jaw, two-week itineraries are Paris to Rome, Budapest to Prague or Krakow, and Spain to Portugal. There is plenty to see to break up the itinerary into smaller parts. Just on the first route, there's the French wine country and Riviera, northern Italy, and Florence on the way to Rome, all accessible by train.