In the eyes of an adult their relationship status seems pretty clear.
First off, lets start with her being a sex worker, is cheating really that bad? At least the sexual part of it? If a person is going to engage in a relationship with a porn star you should probably just consider yourself swingers and drop the whole jealousy thing.
Anyway, So Jon and Christy were in a relationship for roughly a year. Amid plenty of turmoil in their relationship and alleged cheating and abuse from Christy regarding Jon, he moved out in May after a "break up"... one that was apparently initiated by Jon.
So from May, until now... the nature of Jon and Christy's relationship has been undefined. They're on a break, or not exclusive, or just dating again, or whatever. What this mostly amounts to, is that they are still hooking up with each other because old habits die hard (especially in this case) BUT, they are keeping their options open, maybe seeing other people, maybe not. Jon just as easily as Christy could have been banging some chick back in San Diego... in fact, I would hope so.
In this time since May, Christy begins dating Corey which is consistent with what we know about the two of them. So Christy and Corey are hooking up, Corey is extremely aware of Christy's high profile relationship with Jon, he just doesn't give a shit because suddenly he's banging some 23 year old chick that he used to beat off to. Now, I'm sure hot-head dullard Jon has no idea about Corey... mostly because Christy would never be honest with him about it because he has the maturity of a middle-school student "Tantrum Machine" haha.
So lets recap.
Jon - is probably hooking up with other girls too, but is completely attached to Christy. Tweets about her incessantly, has her bumber sticker on his car etc. She's HIS Property... because he's so Alpha

He has no idea about Corey.
Christy - has mostly had enough of Jon over the past year, but she's a 23 year old girl that has her own flaws (boatloads of 'em) So she's incapable of adequately ending the relationship and she kinda still likes bad boy Jon more than her older wealthier douche-nozzle Corey. So she's dating both, because she can.
Corey - is completely aware of the situation, but he's so pumped about banging Christy that he doesn't care... and you know, she's kind of nice in a trashy porn star kinda way. So he's going along for the ride. While Jon is off in San Diego, good ole Corey is right around the block in the playground that is Vegas. He loves bragging to his friends about his Mack exploits and he probably gets earfuls of Christy telling him how she's soooo done with Jon, so he feels like he's moving right on in. Douche.
And there you go, Love triangle complete. If you're 15 years old and reading this... remember it, you'll encounter it either yourself or through friends several times. Just try not to beat the shit out of people and go to jail over it k?
Cool.