I have the same problem! Infact every single time i find myself in a situation for a threesome i just cant get them to agree on one. Have you noticed that the girl you`v been on all night is always the one who doesnt want it, and the friend is up for it?
The question is this, how can you persuade the girl to share your attention?
She knows that if she says no then shes got all the attention to herself so why share?