How to get started making a trimaran

Hi! I'm for a project that would take less than three month to design and build a small (around 12 foot) folding trimaran that I can load onto the top of a minivan. How would I get started designing and making without spending too much

 

Welcome to the Forum 3ert.

For a start, I think you will find a lot of good information here, on Richard Woods' page about small trimarans he has designed -
Sailing Catamarans - Click here for Trimarans Under 20ft (6 designs) https://sailingcatamarans.com/index.php/designs-2/27-trimarans-under-25

Richard's smallest tri is the 10' Tryst; the next one up is the 14' Zeta, so you could maybe try combining features from both for your 12' design?

Although realistically, I think it would be much easier and simpler in the long run to simply choose one or the other to build - Richard has done all the hard work for you already, and his designs are proven to work.