Wow- seriously- polyamory does not work without a LOT of open communication, and it was unfair for him to attempt to spring a surprise threesome without talking to you about it in advance and without letting you have a chance to get to know his other s.o. first. He obviously had this as his agenda as he planned your trip, but chose not to divulge his intentions. That would be a big red flag for me. At the least, he should have mentioned that he'd be interested in a Triad situation, allowed you to react, and if you were amenable, perhaps just making this first trip a chance for you all to see if there's threeway chemistry.
I don't think your reaction is extreme at ALL. I would definitely take some time to process and decide whether or not you wish to continue seeing him. I think, were that to have happened to me, I would have a difficult time feeling the same way about him after that.