Note that the Starman (Thom Kallor) of the revitalized composite Earth formed after the Infinite Crisis states that the original Earth-Two (Earth-2) had thus been reconstituted into the form of the Justice Society Infinty that his Justice Society of America met in the present day, hence the method by which this world (and by extension parallel planets in the Multiverse) could continue to have futures divergent from the composite Earth as seen prior the Crisis on Infinite Earths. In essence, the sole survivor of Earth-Three's universe, Alexander Luthor, implemented the means by which the Multiverse was replicated in a fashion that mirrored the original Multiverse, hence the impeccably identical members of the JSI to those of the JSA and Infinity, Inc. that originally existed. These were the individuals whom Power Girl met upon venturing to their reality from her own adopted realm, carrying on the lives left behind by these Socialites prior to the dessolution of their separate dimensional plane.
The implication of this modern-day matchup between twin Societies is profound, as it leaves the door open to other universe likewise residing, as they had prior to the incursion of the Anti-Monitor's evil machinations. Included in this was Earth-Four - home to Captain Atom and his allies on the Sentinels of Justice, Earth-S - home to Captain Marvel's family and friends, and Earth-X - home to expatriates of Earth-Two that composed the Freedom Fighters. As for Earth-One's reality, this too would need to continue to coexist somewhere amidst the cosmos as the 30th Century Legion of Super-Heroes, of whom Kallor was a member as Star Boy, would, have existed separate of the composite reality composed from the various Crisises. Having been severed from the rest due to its having been the template of all other universes, Earth-One and its reality would have to be rediscovered another time, by a different band of costumed crusaders curiously curtailing creation.