I would never associate the term "throuple" with it, but I think Hutch meant it one way and it's possible that Vivian could take it a different way, because the "discussion" they had about it was anything but clear.
What Hutch said could mean:
1. "I'm open about our relationship being a little unconventional for a while. I don't want to know the details, but you have your meetings and your little sex sessions with Christian, get us through this difficult period, and then in a while he'll get bored of it and we can go back to how things used to be."
Or it could mean:
2. "I'm open about our relationship being a little unconventional for a while. So go fuck Christian, go fuck whoever else you want to, just don't tell me about it, I don't want to know the details. Get through this tough time, get it out of your system, then come back to me and we'll try to move on and go back to how things used to be."
So I think Hutch meant the first one, just Christian, and I think that's how the game will play out. But if it plays out differently, Vivian could understand Hutch to have meant the second one, and think he's therefore giving her free rein to fuck anyone - the mechanic, and the twins and whoever - as long as she doesn't tell him about it. This way, actually, you'd get more of the NTR vibe, because she would be doing it thinking that her husband has given her the green light, meanwhile he would be there thinking she's just out fucking Christian and no one else.
The thing is, I can definitely see it playing out either way, and think the dev would do a good job whichever one does end up being the chosen path.