My experience is limited to Dos Ojos, which I've done twice, but I understand that it is one of the most interesting systems and a very good place for a first-timer. There are two entrances: one at Hidden Worlds and one that is just labeled Dos Ojos. At Hidden Worlds, you ride a "jungle buggy" (2-ton truck with no body) back into the jungle and then enter the system through the Bat Cave, which requires you to climb down a ladder through a hole in the ground. Your gear is lowered down on a line and you gear up on a wooden platform. They try to make it sound fun and adventurous, but it's really just a pain. At the other entrance, you drive back to a parking area right by one of the entrances. Dos Ojos means "two eyes," which refers to two sinkholes that serve as entrances. There is a pool open to the air and then you swim into the system. I found this much more accomodating. The cenote trips most shops set up are two dives in a day in one system, so your 6 dives/3 days plan sounds reasonable. Water temps are a constant 75 F, so a 3mm full suit is usually the minimum exposure protection. A hood is a good idea. Some people like to wear more neoprene. I have heard good things on the board about Diabolo Divers. I went most recently with Yucatek out of Playa del Carmen and they did a good job.
Make sure your buoyancy control is good. You will need to use a frog kick in the cenotes, so try to practice that if you don't usually use it.
