KKK
--2 guys one girl: Easy, b/c guys usually never have a problem with it, you just have to have a girl that is adventurous enough. Ive never had one with 2 guys and a girl, but i imagine the best way to initiate would something simple like joke about it with a buddy and the girl, and then be like,"no, im totally serious

", and she'll probably play along, then keep escalating the joke throughout the night and it'd prob go through.
--2 girls one guy (this is the shit)
a) Style's duel induction massage routine. --look it up, its in the game and involves prokoving kino with the other two people at once and then switching.
-->the biggest thing about these is getting the girls comfortable with each other. Cause usually, one will like you more and wont like the idea of sharing, and you also need pretty open girls who are willing to try new things
b)Sarge two girls that are really close already. If you set your target as a threesome, then pick two girls that look like they already are bff and are really comfortable with each other. Then i'd do the same thing as above, bring it up casually and jokingly, and build on it. Try and bounce with them to your place or wherever, then drop a line like,"so who are better kissers, girls or guys?" Then be like,"prove it" and go from there.
good luck man