Eve, only 3 days? Ugh. A week isn't long enough for me. I've not been to the Yucatan (Penninsula and State) ... yet. It's one of my Mexico goals.
We're going to Mazatlan, West Coast MX the 24th thru 31st of this month. Cant wait. Been there many many times and know it well, going back "home".
The most basics in Mexico? Have FUN! Enjoy yourself. Anymore even the water is drinkable. Do take some Pepto with you, or you can buy it in the hotel or grocery. Nice to have on hand just in case. With as many times as I've been, I've been sick only twice. Once was from the toothless old woman selling shave ice in a small village. The ice wasn't made from purified water, and the shaver was a rusty wood plane. I was bent double for 2 days, and had the stuff for a total of 5 months! The other time was just a simple case of food poisoning from a tourist restaurant. Take and use sunscreen 25 or better. Take a sense of adventure.
Hugs
bren