Every 3some I have had was brought up and initiated by the female I was with at the time. In all but one it was suggested with a '...but I would not know what to do and have never done this before...' type opening as they test your openness to it, and leave themselves room to retreat if you are not.
Few women want their men to know they are more freaky or sexually adventuresome than the men are when in fact I think the vast, vast majority of women are. A lot of men could not handle that truth. they would get very excited a first but then have that moment of 'wait... what' as their imaginations then took them down corridors of male insecurity about what their gal has done prior with other men and women.