I can't seem to find this answer anywhere, i'll feel pretty dumb if someone is able to find it doing a quick google search. But.... I've had not luck. When were the first 3 pin voltage regulator ICs introduced?
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.
The first popular 3-terminal regulator was the 78XX series, introduced, if memory serves, by Fairchild as the \$\mu\$A78XX series ca. 1970. You can find the series listed in the 1973 Fairchild linear databook here, so no later than 1972-73. By '73 they had already introduced the \$\mu\$A78MXX series as well.
The LM317 (adjustable three terminal regulator) also hails from this approximate period.
Possibly preceding the 78xx series was the 3-terminal LM309, a Widlar design, in 1969 according to this. It was not as big a commercial success. I have my doubts about the thoroughness of the research that went into that reference- it does not mention Fairchild at all, and appears to be a National Semiconductor (RIP) centric view of history.
The 78xx series design is one of those that was done so well the first time and achieved such a wide acceptance that they are still used in vast quantities 40-45 years later, despite many competitive parts that perform better under one constraint or another.