Hi Poppy,
I know that usually there is a range of opinions on here, and that normally there is some flexibility about what you might choose to charge.
However I would say that MMF threesomes are absolutely not one of these situations - there's no way you should charge 2 guys the same as if it was just one!! Like you say, the risk of physical harm to you is greater should they choose to turn nasty plus it's twice the work. I feel you should absolutely be charging more than your normal "single guy" rate.
For me personally, I charge double - so they would each be paying my full hourly rate. I do this because they are both receiving a full service, plus the added thrill of a threesome which is normally considered a bit of a "treat" or special experience! So in those circumstances I feel the double charge is fair.
I also feel that if you don't charge extra for more men, then where does it end? If you charge ?100 for an hour, that means that he can bring along 19 mates and they can all pay a fiver each surely. I wouldn't want to leave myself open to that sort of situation.
I think that even ladies that specialise in group sex and parties for many men still charge a fixed (if lower) rate per person, meaning that at the end of the night they have still received far more than their normal hourly rate for one 1 to 1 client booking.
Good luck

Hadley Xxx