Obsa,
I can't cite a specific place to find this out...
I do remember Marlyand DOT making a stink about the two-piece wheel back in the late '70s or early '80s due to a couple of accidents allegedly caused by this type of rim coming apart on the road.
I distinctly remember this making the TEE-vee news...
That said, I doubt there was a Federal mandate; if there was, you should be able to find something out from the National Transportation Safety Board...
Otherwise, you could check with your state's DOT ?
Every time within the last ten years I've talked to tire dealers here in PA about 20" truck tires, they ask me what kind of rims I have and tell me about the 2-piece widow-makers, etc...
I have never gotten flack over the 3 piece wheel ( except from the tire monkey that actually had to change them)
I will check around and see what (if anything) PA has on the subject...
