If you don't like seeing them melt into each other around you, ask them not to do it.
It's really quite that simple. Come up with a code word if you need to, whatever.
It is her dream to have a family with the two of you, so she needs to take a cold shower and be more aware of how she is acting around you with the three of you together. I know, because I hear myself in this.
Mr. A and Indigo and I started having dinner together once a week about a month, month and a half after he and I started dating. At first it was painful. I wanted so badly to be able to touch him ALL THE TIME in ALL THE WAYS. But obviously Indigo was not comfortable with this. So it was usually limited to holding hands and cuddles. I would check in with him during the group time, to make sure he was good, and we would go over any concerns afterward.
It was a lot of work! I was working furiously at being aware of Indigo's reactions at all times, and he was pushing himself to be more comfortable and okay with affection between Mr. A and I.
But it got better and easier. We are at a point now where if I want to kiss either of them, I do it. I snuggle with who I want, and if someone wants more attention, they ask for it or come get it. I don't have the urge to bury myself in Mr. A, at the expense of Indigo. I know that I will have time to do those things with either of them if I just wait a few hours.
So try setting aside some regular time for the three of you together. Cook for them, or have one of them cook for you and the other. I think that knowing that this once a week time was coming up and being able to prepare myself to keep it PG was crucial. Spontaneous things may have been harder.
You guys are doing well. Don't forget that.