Hey can they ask their coding agents to support 3D secure, so I can pay with EU emitted credit cards on the few US sites I'm interested in?

That's for the Stripe customer to configure. Stripe itself has supported 3DS since ages ago.

Edit: also you'll find a pretty common sentiment among US website owners is that the new API that supports 3DS is overcomplicated and they want their 7 lines of code create-a-charge-with-a-token back. Screw the Europeans because they only care about US buyers anyway.