I want these IPs randomized by first Class A, then Class-B, then Class-C.

The other posters seem to have understood you, but I want to point out that you are using those terms wrong. You probably meant "randomised by the first octet, then the second, and then the third". Class A through E have to do with networks and subnetting. A class A network would be a network of 16 million IP addresses, and its address would be somewhere below 128.0.0.0. (Since the introduction of fine-grained subnetting, these classes have almost lost meaning.)