How about one text/email message to all the students telling them that if they miss three lectures or one exam they willFAIL the class?

I know this is awfully politically not correct, seeing as how 99 & 44/100 % of all students feel entitled to at least a "C" merely for enrolling in a class, but hey! I'm a big believer in the concept of dumping the deadwood at the first good opportunity.