I think you show a great deal of empathy and understanding for your girlfriend. I agree with your first statement, you shouldn't do anything unless she's enjoying it too. It sounds like your girlfriend was made to do things in the past that she was uncomfortable with and you don't want to repeat old mistakes.
She Is Willing To Do It
On the other hand, if this is something that you both want to do for you as a gift or something, and the third person knows what she's doing (you're not forcing it on her or anything) then why not?
Mutuality Is The Key
I think the key here is mutuality. You both should feel that you are doing it for the right reasons and you both are enjoying yourself.
I should let it be known that I have had very little, if any, real
recreational sex. It's not that I disapprove, I just haven't really had the opportunity. I also never wanted to damage the relationship I was in.
Is It Worth The Risk?
I think that whenever you involve someone else sexually, you are taking real risks with each others feelings and the trust you have in your relationship. You have to ask yourselves, it is worth the risks?
Also, I wonder (since you both seem very experienced) why you want a threesome? Why are you bored in your relationship if you are still discovering each other and what turns each of you on? I highly doubt that having a threesome will make you closer, but again, I've never done one.