From my understanding. 0.5mm have to be mill out for suspension arm blocks and gearbox area for 3mm chassis is base on the ride height measure affect on roll center location. As far as I know, the standard chassis bottom thickness is 2.5mm, if making 3mm chassis for it and have to remain the same ride height as standard chassic, those suspension arm blocks and gearbox + shock tower will be mounting 0.5mm higher, and the most important thing is the central gravity will mount 0.5mm higher as well.
From my point of view, CG height is the first piority to decide before chassis stiffness and suspension gemotry. We always looking for lower CG but those roll center is being able to change if we want to, such as putting some shims under suspension arm blocks to give higher roll center...etc.
2.5mm chassis base combine with top deck and side brace should be stiff enough and no scratching the head for remain on suspension gemotry.
The only things might possible to improve is try to push all the batteries and electric stuffs closer to the center so all our TC3 owner might have a better responsive car than normal chassis.
Once you finish the project, will there be a chance for me to purchase one copy of you?? good luck with it.