User blog comment:Curtis712120/Earth-3/@comment-28458080-20160623042720/@comment-27700324-20160623173835

The problem was that we didn't see much of what Central City on that Earth is like, also Barry said:

"You have Central City but you don't have S.T.A.R labs, no Cisco Ramon, no Catlin Snow, no Harrison Wells."

This means that there was no particle accelerator that created the Flash, Barry probably didn't research into it because science dictates that the Flash can't exist on that Earth. However, he could have been created by what Hunter Zolomon said to have made him into the Flash, the experiment in his lab. So here is the summary of the origin that i presume to have happened.

Jay Garrick was working at his lab and got tired, the chemicals he was working on splashed into him, turning Jay into the Flash. Supergirl doesn't know about him because as we know, he was taken from his Earth by Zoom and left in a cage as a trophy. This was around the time of Kara's career, we don't know what will happen next, but possibly Supergirl will meet the Crimson Comet.