So, he cheated on you with her.
You guys had all talked together and come up with the structure of how your relationship would work, and he (and she) violated those, not by accident, but deliberately and often.
The open and honest way to have gone about this would have been for the three of you to sit down and talk about how he was having feelings for her, and that he would want to spend some time alone with her and, so that it wouldn't take away from either your time with him, or your time with her (or with the three of you together) he would rearrange his work schedule to make it work. It would have been something that you could have discussed, each could have brought up concerns about it, and you could have reached an agreement about the change before proceeding.
So no wonder you are feeling hurt and that your trust has been betrayed.
Time to get things back on track - I really would suggest that the three of have a sit-down and talk about your evolving needs and concerns, and try to work together to find out whether there is a solution where everyone can be happy. Also, you need to reaffirm that any changes that are desired can be discussed openly and honestly between the three of you, rather than anybody going behind anybody's back and lying.
"Listen, or your tongue will make you deaf." - Native American Proverb