Hi,
The title of your problem is not complete. It should have said, Messed up sexually, mentally and morally.
The solution to your problem sexually was not for your wife to cheat on you. The solution to your feeling good sexually is not to have a three-some. Now, your wife does not like the idea of the three-some so this is a problem for you? I just don't understand the way you think. You both are not doing the right things to make your marriage work. Instead, you're adding more problems to the existing problems. How can you solve this problem?
I do think that you both should see a marriage counselor and find out if your marriage could be saved. This would require for both of you to return to your marriage with the thought of being monogamous, no other man or woman to come in between you.
If you both could not go back to monogamy, then even the best marriage counselor won't be able to help you.
You both need to change to make your marriage whole again.
Take care.