I know what you want is threesome

I have been together with my girlfriend for three years, we love each other. And recently things become different between us. She is not that excited when we are having sex. She seems less high and I don't know why. One day, she visits a tinder for threesomes dating site, and I know that she wants a threesome dating. It is a little surprised. Well, I would like to try a threesome with her to make her happy.