I have a couple of trailers that would do that job with no problem. Actually, they'd accomodate a minimum of FOUR. I'd suggest you look at something like an 18' solid floor car trailer with extra tie down points (you can never, ever have too many) and wheel chocks installed. If you're willing to live with a little angling of the third of three bikes you can probably get by with a 16' trailer, which is a far more common size to find used.
If you need any help setting this up or have any other questions, feel free to PM me as I've done a lot of trailering of vehicles of the two, three, four, six and ten wheeled varieties over the years.
Thanks to all of you, even the wise guys. I think I enjoy the humor on this board as much as the info. Just for the record, the reason I, and a couple of my friends, are looking for a trailer to tow our motorcycyles is because we also want to take our tandem bicycles along. We have several destinations that provide good riding for both types of two-wheeled conveyences. We thought about towing the bicycles behind the motorcycles, but it all starts to get pretty complicated given the length of the tandem bicycles, all the specialized clothing for both rides, etc., etc. We concluded trailering all the bikes, motorized and not, would work best.