Either a marconi or lanteen rig
You may be thinking of the mizzen, which would be the third mast. The third sail would be named based on the specific sailing rig, and on the arbitrary numbering of sails. For instance a brigantine rig could have 11 sails, the third one could be a topsail a gaff-rigged mainsail, a jib, a course, a fisherman etc.
You might be thinking about the sailing vessel known as a "clipper", a very fast sailing ship of the 19th century that had three or more masts and a square rig.
Example sentence - There is much to learn about the rigging on a sailboat.
Brand new offical laser full rig sails cost about £525 (ca. $780USD)
Trapezoidal, actually. But the square sails were called square sails. Lateen or square could be referred to as yards.
Getting big rig insurance can be very difficult to do. The best place to look would be the website Allstate.com.
Stealth: At night, they are more difficult to see.
You can visit www.TruckCoversDirect.com for a semi rig cover. You shoud also check out the selection at Pulltarps.com.
Most sailboats have white sales. One could have black sails, but it would quickly fade from being out in the sun.
Square sails are square
Square sails are referred to as four-cornered sails.