Boat hulls are commonly made from wood, steel, aluminum or fiberglass. The choice of material is dependant on hull design, intended use, production facilities available and, to a lesser extent, cost.
In ancient times (and to this day in certain places), boats were made from bundles or reeds tied tightly together. Check out the boats used at Lake Titicaca or on the Nile delta.
Now if you are asking for hull shapes or types, that's a different question. See other QA in this catergory, or use wikepedia for a detailed (and complicated) answer.
A Trimaran is a boat with three hulls.
There are two different elements to a keel. They are the hydrodynamic element and the structural element. In layman terms, the keel is the backbone. There many different hulls. When you speak of hulls, you have displacement hulls, semi-displacement hulls or planing hulls. Hulls come in different shapes like moulded or hard chined or simply chined.
A trimaran has 3 hulls, a main hull and two smaller outrigger hulls.
Boat hulls are currently usually made of fiber glass. Fiber glass is the cheapest, most effective material. Boat hulls can also be made of wood, carbon fiber, and steel.
This type of boat is called a Catamaran.
A trimaran is a sailing yacht with three parallel hulls.
There's no law against it, but it will slow your boat down.