I think they will probably have at least one more threesome where the mc actually fucks Elena, but yea after that I can see john boy going full cuck mode and just watching.
I know its a porn game... but the game doesnt follow THAT many porn-fantasy rules... regarding this its kinda "tame" and is following more the logic of the "world" the game is set in and how the characters behave.
A bit of sharing and involving another cock is one thing... but letting the MC fuck Elena for real is a totally different thing. And would potentially ruin their relationship. I dont think DPC is planning that. And we all have to keep in mind that ep 12 of season 3 is more or less the last episode like we know the game. Its not really the time aymore to start something like a full on cuck route for John Boy... because there is no time left to play it out anyways.
Was it a total one time thing? Hmmmm, i do think so. there might be another "try" but i dont think that Elena and/or the MC would go through with it to fuck before John Boys eyes.
DPC made it very clear that they love each other, and i dont think he want to change that. And between Elena and the MC is a pretty good and healthy friendship too, which would be over if they would go through with that... so no... i dont think there will be a cuck route.
So my guess, if they will try to go that way, and try it out that MC will fuck Elena... Everybody will get cold feet before its really happening... and thats it. Not every Pornfantasy will becore true in this game, this is one of those.