I would basically explain exactly what you did here. I would just say that in the interest of being open and honest and trying to make things work, you want him to know that you slept with these two friends, tell him when it was and roughly how serious it was, reassure him that you truly have zero romantic interest in them and they don't have any interest in you, and see how he reacts. Personally, if I were him, I would be mildly concerned at first but over time if I see your boundaries are good and you are trustworthy in general throughout the relationship I wouldn't mind at all. Realistically, this is part of your life, so getting it out on the table like you are doing is a good move.