X-Eros is the most "hardcore" of the three. It has more of the hardcore fetishes like netorare/netori, mind break, hypnosis, blackmail, etc. In the grand scheme of things, it's still mostly vanilla.
Shitsurakuten generally (very generally) has older women, or to be more precise, older-looking women. So basically Shitsurakuten's high school girls look older than Kairakuten's high school girls. This is a very hazy distinction.
Kairakuten has a lot of vanilla.
Kairakuten and Shitsurakuten are supposed to be a bit more "family-friendly" or whatever would actually make sense here, so they are subjected to the so-called "lightsaber dick" method of censorship in Japan, while X-Eros has black bars. Our releases are all uncensored so this doesn't directly affect you, it's just another small difference.
X-Eros usually clocks in at around 500 pages, Kairakuten at 400, and Shitsurakuten at 300.