Well, the two girls I did the threesome with, neither one was my gf like I said. And, also they were best friends and have known each other since they were 15, so I think they have experience at that because it sure didn't hurt their friendship.
You asked her if she wanted to grab coffee. Then you asked if the time worked. Then you asked if the place was okay. Then you asked if she'd prefer tea.
Four questions. Four chances for her to say no. Four signals that you're terrified of making a decision.
Plan it. Pick the place. Pick the time. Tell her where to meet you. She'll either show up or she won't.
Either way, you look like a man instead of a scared little boy.