a three mast sailing ship
a vessel with at least three masts, all of them fore-and-aft rigged , except for the foremost one, which is square rigged
A vessel with three or more masts with square sails on the foremast, and fore and aft sails on the main and after masts. Generally in the 250 - 500 ton range.
Ships , Rigging Barratry Expressions