the route back is faster
Back to sensibility:
By my estimations (ignoring the cutscene at the very start) the point where u can cross campaigns is all the same time frame, after that the time frames break up. Tryia and Elona occur at the same time, However the there is a large time frame where the Sunspears gather there resources attack the Kournas and then another long period they hide in the sanctuary (a several year period) so by the time u complete Abaddons Gate it is indeed 3 years after of the events tryia and Elona.
One of the reasons for this is the fact events up to LA, Kaineng and Kamadan are proclaimed as having already happened (as seen why a non indigious char attempts missions earlier) however Eloan chars who go to LA dont get this message from LA onwards missions etc. meaning those events havnt occurred yet (despite the 1st Elona cutscene claiming its 3 years ahead)
This intersection is when the heros choose to go down a pathway to save a continent, and head off on ship to Elona, Cantha etc. (or stay where they are)