Originally posted by Eryk Davis Ester:
I believe the basics of the Legion origin (with the founding trio) was actually first established in a letter column or text feature, though I can't confirm this, since I rely on Archives for my Adventure era knowledge.
I just gleaned these references to the Legion's origin from The Legion Outposts which can be found on the site below:
Adventure Comics No. 326 (Nov. 1964)
THE LEGION OUTPOST
Dear Editor: I would like to know who started the Legion of Super-Heroes. And how did the other heroes know about the Legion? -Stephen Rich, Staten Island, N.Y.
(Cosmic Boy, Saturn Girl and Lightning Lad were the first three Legionnaires. The news of their super-deeds soon reached every corner of the universe, and other super-youths were eager to join them. -Ed.)
Adventure Comics No. 352 (Jan. 1967)
THE LEGION OUTPOST
". . . Finally, when are you going to tell us the origin of the Legion - how Rokk Krinn (Cosmic Boy), Garth Ranzz (Lightning Lad) and Imra Ardeen (Saturn Girl) met; how they decided to form the Legion; how other members joined them; the origins of their costumes and titles - everything? It would be the biggest hit of the year! -Leonard Rosenberg, Bronx, N.Y.
(We'll be getting around to it, Len. Meanwhile, you'll find a brief resume of the Legion's start in "The Origin of Cosmic Boy," which appears at the end of this column. -Ed.)"
KNOW YOUR LEGIONNAIRES
THE ORIGIN OF COSMIC BOY
. . . Rokk decided to do something to help his family. He figured his magnetic powers would be useful in some industry on the planet Earth - so that's where he headed.
It was just at this time that two other alien young people with super-powers chose to visit our world - Garth Ranzz and Imra Ardeen. Garth and Rokk met soon after they reached Earth and they roomed together. One day, they happened to be at the same place as Imra - and the three of them used their powers to save the life of the richest man in the universe. So impressed was he that he suggested they form a crime-fighting organization, which he would finance. This was perfect for Rokk, since their benefactor, Mr. Brandes, included ample money to support them and their families. The full story of the Legion's origin will be told in a future issue. Suffice it to add that Rock, Garth and Imra became, respectively, Cosmic Boy, Lightning Lad and Saturn Girl, and they have since used their magnetic, electrical and ESP powers to battle evil throughout the universe. . . .
Adventure Comics No. 354 (Mar. 1967)
TEXT FEATURE
MEET THE LEGIONNAIRES
LIGHTNING LAD AND LIGHT LASS
. . . Soon after, Mekt disappeared. No one could say where he had gone or why. Garth set out to find his brother, and his search led him to Earth, where he ran into a dead end. However, one day, as he and a new friend, Rokk Krinn, who had come to Earth looking for work, were walking along a street in the future Metropolis, they heard a blonde girl exclaim that two men were about to assassinate a prominent tycoon. Quickly, Garth blasted one assassin's gun with a lightning bolt, while Rokk used his magnetic powers to disarm the other. The girl, Imra Ardeen of Saturn, had read the killers' minds! So impressed was the intended victim, R. J. Brandes, richest man in the universe, that he suggested the three form a crime-fighting combo, and that he would underwrite all their expenses. Thus the Legion of Super-Heroes was born. Garth became Lightning Lad, Rokk took the name Cosmic Boy, and Imra called herself Saturn Girl. No doubt Imra was a major factor in getting Garth to join the group. She won his heart the first time he looked at her. . . .
http://www.studiosanning.shawbiz.ca/legion_of_super-heroes/legion_outpost/index.htm