I think it would be best, though, to check fares for the timeframe you are interested in to each of the destinations. Since St. Thomas and St. Kitts are much more seasonal than Aruba, depending on the time of year you may see large fare variations.
You can easily run the fares on Kayak.com for when you want to go and make an easy comparison. Kayak, btw, gives the full fare (inclusive of taxes) while the above link doesn't include the taxes/fees.
Also- although you didn't ask- again depending on when you want to go- don't let just airfare determine the destination. For example, if you are travelling during hurricane season, there is potentially a big difference between islands.