Boyfriend's friend asked me on a date??

Not officially my boyfriend yet but we've been going on dates for about 3 months now and things are starting to get pretty serious :) anyway, one of his friends asked for my snap and I seriously thought it was just going to be a platonic thing because I've known him for a bit now and we always kid around, but he asked me to go see a movie with him... I replied and was like "thanks for the offer but I don't think that's a good idea?" And he said "that's fair". But like what the hell, do I tell the guy I've been dating? These two are apparently really good friends and also teammates (we're in college btw) so I don't want to completely wreck their friendship which makes me not want to tell, but at the same time I don't want him to think I'm being sketchy or hiding anything? Wtf do I do?? We were talking about a movie coming out and I said my guy probably wouldn't go with me because he doesn't like scary movies. Then his friend literally said "next time you're by ("boyfriend's college") maybe I could join you to see (the movie we were talking about?"