Is "number of runs in a game in the first three innings" also a piece of stats?

Sure, it’s a statistic - by definition. But no, it’s not a commonly used statistic, and wouldn’t appear on a BBRef player page. Baseball statistics like that are collected either by the sports department of the TV station, or by ESPN which is an entire sports TV station, or by a variety of other organizations; the Society for American Baseball Research has a list of examples as well as being one of those organizations itself.

In this case you probably could find it with a baseball reference advanced search for team stats - they didn’t mean Devers scores three runs on average in the first three innings (that would be not only a record but probably by a factor of four or five; I doubt anyone averages much more than one run a game) but that the whole team does.