Not so. I did one with a regular and a girl she works with. I had a 121 with her friend first, then booked the pair of them and gave a very specific agenda. It was comfortably my best punt to date. Too many bring random girls together and then just turn up without a plan. No wonder its rubbish.
I've had 3 threesomes and by far the best was with 2 WGs that I'd enjoyed punting with individually (Emma & Amy in Milton Keynes) plus had checked with both that they were happy to go interactive and snowball my cum, thankfully they were good to their word.
The 2nd was a spur of the moment decision when I'd booked to see one British indie on AW and caught a glimpse of her more attractive housemate who joined in for a 3some but they didn't snowball or have any double-headed toys to use on each other; I would probably have been better off booking the looker on her own in hindsight.
The 3rd was with a couple of Romanians (before I began frequenting UKP) but most of their advertised services turned out to be somewhat spurious, thankfully they were cheap and I've yet to receive a double-blowjob that I didn't thoroughly enjoy.