The OP isn’t specific enough.
Is the question, “is it plausible that this might actually have happened” sometime, to someone?
Of course. Practically a certainty.
Is the question, “is it plausible that this might actually have happened” specifically to the person who made this claim?
It’s a longshot, but not such a longshot that it can be dismissed out of hand, unless we have additional info about the credibility of the poster.
“Yes Virginia, there are pity blowjobs and three ways. And bridges for sale and Nigerians who’ll give you money to help them transfer funds. Now pull Santa’s other finger.”
Completely believable to me. My cousin and her husband wanted me to join them for some fun and if I hadn’t been only 15, I might have taken them up on it.
It was a long time ago and they were only 18 themselves.
Not credible to me. Might have happened to RDJ when he was 20… or maybe Mel Gibson when he was 20. Not average guys.
Its right up there with guys trying to stealth-brag a 10-inch johnson while their ex GF shakes her head and mouths the words “Five Plus Money. Maybe.”
Only if you’re trying to answer some question the OP didn’t ask. 
Just for grins, you ought to call them up and say, “Hey, remember that threesome you suggested when we were teenagers? I think I’m finally up for it now.”
They’re divorced and have each remarried, so it would be a larger group. 
And that’s not what you’d expect.
Completely believable. First I used to know someone to whom it happened (asked me if I thought it was a good idea, and I was myself interested in this person for an added layer of complication) second because I’ve seen weirder things than that and third because I’m well aware that when sex is involved, you should never assume that what you’re familiar with (directly or through your self-selected sample of friends or exes) is terribly representative.