I would like to have a long-term open relationship

I have a girlfriend that we have been together for three years. We love each other so much. While, I wanted to have a threesome dating for a long time. But I don't know how to ask my girlfriend to say yes. One day, she saw me talked with a girl on 3rder(the leading tinder threesome dating app). She said if you want to have a three way, let's do it. So, I asked the girl out and that night was amazing.