i have no idea why she had a 3 way , but have you considered that it may have happened because she had not had a boyfriends before, and just wanted to feel accepted. i personally am not going to have a 3 way because im not into some creepy couple that cant be in a relationship by themselves and need to drag some other chick into it to get off.   why is the girlfriend not enough for the boyfriend? why is the girlfriend okay with sharing her boyfriend , why would she be okay with her boyfriend needing to be sexual with someone else , and make her participate ? i could be completely wrong about the motivations of this but personally i feel that a young female without any real relationship experience , likely had/has little self value , or maybe does not have the courage to say she isn’t interested, and ends up doing something that she may not regret at the time, but  eventually may impact The future connection with someone who wanted to care . its hard to respect someone when you know that they haven’t respected themselves, but maybe this is not the positive experience that they are all making it out to be , maybe they don’t know how to form healthy attachments and mix in the influence of todays   everything goes culture. maybe people end up doing things that might feel good then but with time and maturity are not something they are particularly proud of or would do again. and i am not a prudish person by any means. but in my personal experience , sex with out feelings or emotions feels like your being used, amd it really only takes 1 or 2 times for you to be able to KNOW this. so if you keep doing this , you are kinds a sell out, wich in turn lowers your value , cause if u are willing to have someone touch your body and touch theirs in return that , you cant claim to have a real connection or desire to be with , then the only reason your doin is would to gain something rite?  what is the gain what was the price for you self respect and your body ?   none of this is specific to this situation , they may have completely different mind settings than me , but if she had similar thoughts, the fact that she did it is not the bad thing, it would only be bad if she feels similar and continues the same behavior.  if it makes u feel dirty , then lesson learned , now she knows what not to do and u cant hate on that but f she feels dirty but continues to repeat the same actions with the same or other people whether it be a 3 way or not so casual for her only sex, then she may have some attachment issues that are not your problem unless you want then to be, having said that , Goodluck  finding someone who had not “sold out ” or has healthy attachment style.   lol that was not very helpful hu? sorry , hopefully i was able to put into text what i wanted to get across ,i have difficulty doing that most times. i think I’m trying to say , if she does not feel good about what she has done before, but now knows that she does not want to pursue that again because she has experienced good with you , then you should try to forget it little harder, but if she has negative feelings about what she has done and continues that behavior , its a cycle that is not gunna stop until she stops it. on the other hand if she feels no negativity , than that is just how she feels and you cant fault her for it but maybe your values are misaligned and no amount of trying to make it work will fix that. for reference i am 30 female and just married my bf of 9 years  soi may not relate yo your age and life  .   hope you figure out something that you can be happy with  i say this alot but seriously look up  attachment styles and get insight on why you form the attachments that you do .