@Jan,
Not my personal cup of tea, but I have had buddies who did these, and I've heard of a few duos in the seduction community over the years who specialized in them.
The claim by the guys who do them a lot is that loads of girls are down for them, and that it's easier to pull many girls for a MMF threesome than it is for a one-on-one lay (due to it being a novel experience for the girl that many/most girls have fantasized about).
I have personally had many women admit to when asked that they would absolutely be down for a MMF threesome. If you haven't talked to girls about this, you might be surprised how many girls say they'd like to do this if you're good at getting women to be candid about their fantasies with you...
Anyway, can't vouch for the effectiveness of going for MMFs personally, so this is just hearsay from me... but it's an intriguing claim by the guys who go for them regardless...
Chase