On a catamaran, you simply measure centerline to centerline front and back, and you’re there.
On a trimaran, if the amas have no angle, or if the sheer line is parallel to the water, the same.
But who wants amas with no angle and no shape on the sheer? On these, if you match centerline forward with centerline aft, your hulls will not be parallel. This slightly ancient graphic shows why. The CM Construction Manual has this and the new Composites Manual underway now will have a newer graphic of the same thing.

I just got some pictures of this sweet little KHSD 23′ day trimaran. Nice job. From New Zealand.

I’m working on this little 17′ roto molded beach trimaran. It is intended for resorts.

I found this beautilful old style KHSD D-30 demountable trimaran on the web somewhere.  I have no idea whose boat it is. I see it has the old style spade rudder that could kick-up even while turned.

I got a picture of this plywood 37′ trimaran the other day. Database told me the plans went out in 1990 to Bora Bora.
It looks great.