So was it more of feeling left out because they started the party without you? Or are you upset by seeing your best friend and your husband making out?
I think your heart is broken because you really didn't want to do it in the first place and seeing them kissing put that right in your face.
I have to ask, did they have to do some persuading to get you to agree to the threesome in the first place? Saying it would be all about you? Obviously that wasn't the case.
There is nothing to forgive... learn from it. You are not comfortable sharing your husband and you shouldn't be drinking to the point where you make decisions you are not fully comfortable with. Have a talk with your husband and your friend and make sure they understand that it won't be happening again if you aren't 100% sure it is something you want to be a part of.