I would check to see how close you are to the stations as this would help you decide. If the 3 that you can get in are like 10 miles away, but the 3 you cannot are 20, you might need to get a full sized antenna. You might be able to just get an amplified set of shorter antennas that will do the trick.
http://www.checkhd.com/aw/welcome.aspx
I'm about 40 miles away from all of our stations, and we have to have a large antenna with a booster in order to get everything in.