Why we bought 3SUM

First, trimarans can move a lot faster than a monohull of the same length. Speed is important in a cruising sailboat. Being capable of higher speeds allows you to cross long distances with fewer supplies. It also provides a measure of safety when dealing with bad weather.
Second, the storage space on a trimaran is greater than that of a comparable length monohull— on most designs the amas can be used for storage.
Third, multihulls are much more stable than monohulls— this is due to the greater beam afforded by the multiple hulls. The stability is provided by the amas, not by a heavy keel. In most trimarans additional stability is provided by a centerboard system.
Fourth, multihulls with a centerboard design can have a very shallow draft— which allows you to sail in waters not accessible by the deeper draft of a monohull. In a cruising boat, this can be important— trimarans are beachable, so you can go ashore on small islands that don't have marina facilities.
Finally, the ballast-free design of most modern trimarans makes for a much safer boat. Some of the newer multihulls are so buoyant that if you filled the hull and the amas with water they would still float.