With it being a Sat night, her expecting to go out with you, have drinks, and then leave at 9:30 or so to go to a movie (which is probably with another guy), which in the meantime leaves you hanging right when things get going? Not the biggest high level of interest sign I have ever seen, and kinda rude. I would cancel.
Say something like this. "Hey, didn't realize you had another engagement to go to that night. I had dinner planned after drinks so lets reschedule for another night when we both have more time. I have some friends that asked me to join them so no biggie. Have a great time at the movie!"
I bet her jaw drops when she reads that. She may even offer to cancel the movie. And if she does? Tell her no, that you already promised your friends you will be joining them.
Also, no doubt she is spinning plates. Seeing other guys. That is why she flaked twice before, was late on getting back to you about Sat night, and why she has to leave early to go to a movie. You should do the same. Spin.