When were 3-pin voltage regulators introduced?
Bob Widlar designed the first monolithic IC linear regulator, the µA723 for Fairchild Semiconductor in 1967, and later designed the LM100 for National Semiconductor.

This is the same guy who designed the first monolithic IC op-amp, the µA702, also for Fairchild.

Sadly, you're not going to find many online references to these things, since they're all long-obsolete, replaced by jellybeans nearly as ancient.

Now, the 723 isn't a three-terminal regulator. It's adjustable and more complicated to use than the later LM117/317, which essentially replaced it. The first monolithic three-terminal fixed regulator is probably the LM109/309, released in 1969.