Advice Needed

Yeah... so she knows you like it. Do not need to inform her again.
She is just only a little bit into it. Just very little and much less than you. So I think not much to do. Some encouragement... but she'll get the feeling you are pushing.
 
This can only be one thing. She is concerned about what this does to her marriage. Why is of little concern, if she won't come out and tell you or discuss it with you. You will have to make her feel this is a normal healthy integral part of your marriage. She needs to understand that you love her in every respect.

It would be helpful to know what she is thinking right after one of the threesomes, but not as important as the above statement. But to ponder that; ask yourself what did you act like before, during and after the threesome. Chances are you were not as loving and reassuring and conversive about what just happened and how it fits into your love for her.

Plan:
Pour on the love and affections for a week. Then bring up the subject and begin a dialog. Shower her with praise for all the positive things she may relate to you about it. Deep dive into the negatives, weed out any misconceptions about how you feel toward her that you can find. Be patient, she will bring up things that you had thought were done, and more than once. That means she is not convinced it is something she can live with.

Women do not think like men. I can't say that too many times. Whatever you think she is thinking, you're wrong. You may not be, but you should assume you are wrong, until you hear it from her. If you can't find out from her, don't act on your thoughts. Rather pour on love and affections. A woman will do amazing things for her husband, if she thinks it will please him.