3 seconds is wrong. But there was an observation made in the late '70s, when sub-second response time was hard to get (but also when a lot of editing was done locally by the local 3270 terminal - so many trivial interactions, like character editing, really were zero response time) that people would get used to sub-half second response time and then the bar would be raised.
Maintaining sub-half-second response time was very hard in a cyclical environment, like a university. Between assignments it was easy - the day before the assignment due date, it was just not possible.
The comment was made that predictable response times were a better human factors choice than unpredictable ones. The suggestion was made that there be a minimum response time, settable by some kind of configuration parameter, to better ensure predictability. I don't believe it was ever a hard wired fixed time, such as the poster suggests. And I believe it was an option you could turn on or off.