I don't think you'll find anything in 3 cylinders till you get over the 20 hp mark.
FWIW (not much) I did have a Perkins M18(18hp) early 90's vintage that was a 3 cylinder, might have been a detuned M20.
40' dive boat + 200 lb. crated engine, any chance you can self import ?
Also, A friend that was a baggage handler for an airline used to complain about a weekly red eye to SA that had no limits on checked baggage. So he said people used to wrap suitcases around engines, appliances et al and he had to load them. Maybe a just an airline urban legend now.