Eeeh.... threesomes always sound good in theory but they can be tricky. Often, one person gets more attention than another and it can be the case that this person is often the one who is the newest addition.
It could be that your boyfriend is not as interested in you sexually, that he doesn't care if you have an orgasm or not, that he is really interested in this other girl. That's where your fear obviously took you in the moment.
Or it could be all manner of other things. Your boyfriend could feel incompetent at making you come and has stopped trying. He could have thought it was turning you on to watch the two of them. He could have been simply trying to 'perform', give this other girl a good time whilst not realising he was causing a reaction in you.
Someone on here (GalaGirl) gave me a great analogy about poly. She said it's like a water bed. You're comfy enough and then someone adds extra people to the bed. It bobbles about and people become uncomfortable. If the water bed is already wobbly, it's not a good idea to add extra people to it at that time.
If you have some sexual issues to work out... it's not always the best idea to add an extra person to the mix. If you feel sexually unfulfilled or neglected, you have to address those issues. Like anything in life, it's safer and better to be in good solid place before you shake things up.
Talk to your boyfriend... tell him why you had the response... don't accuse him or shout at him... try to talk it through calmly.
It's not ideal that you asked them to stop... but emotions happen. Sometimes we just cannot bear to stay in a situation. It would have been better to remove yourself, rather than asking them to stop, but we all make split decisions in the moment.
Take some deep breaths... think about the underlying issues... and communicate with your boyfriend... once you understand each other, look at practical ways you can work better in the future. Agree on whether or not it's ok to stop a threesome in the middle of it and how either of you could best go about it if you need to step away.
me: female, 29
GF: my primary girlfriend, 39
3 year, open poly V, long distance