I would think it depends on what type of S&G you are thinking about. I have a kayak made of 3mm plywood. It's sturdy because it's tortured ply and the all the panels are curved making it a stiff tubular unit. Don't know if I would try making a normal S&G configuration from 3mm ply.
Since you are contemplating a totally new 3mm material you need to know what it's engineering characteristics are. Stuff like stiffness, impact resistence and weight are important.

: I've been doing a little bit of work with clear acrylic of late, and spied
: some full 3mm sheets at the local supplier.

: Messing with solvent welding of the stuff caused me to wonder whether anyone
: had attempted to make a transparent S&G style kayak from anything similar.
: Careful welds, and a bit of careful sanding and polishing should yield
: quite an interesting (not to mention unique!) boat.