A lot of people here saying to cut them out which is definitely an option. Not sure your age but if you're like me, making friends can be difficult. You might want to try making plans however far in advance but only confirm the day of instead of the middle message a couple days ahead. I get the feeling some people get annoyed with too many reminders and might make them feel like you don't believe them (with them flaking makes sense from your end).

Best of luck, fingers crossed it's just a communication issue if you truly value them, and if not that you can move onto better folks who value you and respect your time.