I have always loved the idea of a threesome and hubby and I have discussed it on and off. The reality is a complete turn off though.
I could never enjoy watching hubby pleasure someone else and I watched too many Jerry Springer episodes where the guy has left his partner for the other woman in the threesome. As they say, 2's company. 3's a crowd.
If you just have to have a threesome, I suggest paying someone to be the 3rd party.
As for getting over your reservations, I don't recommend you try. You have reservations for a reason.
One last question, are you bi? If not I don't understand why you want a threesome.
'People are never perfect but love can be. People waste time looking for the perfect lover rather than creating the perfect love' - Princess Leigh-Cheri from Still Life With Woodpecker.