The timeline in this story is a bit odd. It begins with a framing sequence, where the Legionnaires visit a planet in the Antares system and find a statue honoring the Unknown Legionnaire. Superboy asks if anyone remembers that adventure, as if it was long ago. This isn’t the first time it’s been made to sound as if the Legion’s adventures have been going on for the seven real time years the group has been in existence. Age-wise, Superboy hasn’t aged out of high school yet, which puts a pretty short span on his Legion career since Adventure #247. Three years would be an outside limit, and I say it’s pretty outside.
More, at the beginning of the flashback tale, Supergirl talks about getting back to school at Stanhope College, which she started attending in the November, 1964 issue of Action Comics (#318) —less than a year ago in real time, and probably much less on the Legion’s timeline. So the flashback “do you remember” device is glaringly off.