First, have a guess and then decide the ways. If you have been with your partner for some time, I think you would have a general impression or a probable guess of it. Has she ever talked to you about kissing another girl or something like that? How does she feel about lesbians? I bet you have talked about such issue in your daily life. You might not pay much attention to her back then, but try to recall her attitude towards it. If she has kissed a girl before, or has a supportive attitude toward lesbians, you can take a more straight-forward way to talk to her. If not, you might need to take some detour.
If you think she is okay with it, all you need to do is just talking to her about it, but please do it at the right time. Do not say it when you see some pretty girls passing by. This might be a suicide. You can say it when you are watching a threesome porn or at night after you heard a funny tinder threesome story from your friend. Don’t make it a huge deal. Just start a casual conversation about it.
If you are not sure about her attitude toward threesome or you sensed that she might not be okay with it, there are ways to change that. First, she needs to be 100% about your love and she needs to trust you with all her heart. This is your job and no one can help you with that. Don’t ever think she would agree to it just after a big fight. In other words, your relationship must be steady and she must feel that way. Second, get threesome hookup more exposure. How can she agrees to it if she doesn’t know what is good in it. Tell her more about threesomes. You can watch threesome stories or threesome movies together. Get her curious. Curiosity can be your best assistance.
After you get her curious about it, you can go back to the first step. Choose a right moment to ask her. But try not to be too direct, such as “I want to have a threesome.” This is not a nice request, but more like a threat. Try words like “Honey, how do you feel about kissing another girl?” Or “How do you feel about us having a threesome?”