Threesomes are very tricky and don't usually turn out, compared to what the participants fantasize or see in porn videos. In order for a threesome to work the couple needs to equally want this, be very secure and willing to give up jealousy/possessiveness.

It seems like your GF wanted this and she didn't want rules even though you did, therefore I don't think you should be blamed and called a cheater. I think a couple needs to be prepared for mixed feelings when its happening and she obviously was having this. She should have been honest and called a time out or agreed to the some rules as you suggested. Threesome rarely work as well as the videos depict. The only time it might work is, if all three are casual acquaintances where there's no emotional attachment. I don't think your GF can accuse you of cheating since she didn't want any rules - what were you supposed to do...be like a statue?