3 may be too young.. I know DD was 7 when she did it.. we did the encounter where we got to ride the dolphin.. there may be other encounters that maybe a 3 yr old can do.. You might check into it and see.. I cant remember the name of the company but if you go to cruisecritic and search for cozumel you will find much info and the direct website where you can contact them and ask....
We took a taxi to the park since we booked ourselves.. it was much cheaper than what the cruiseline charged for the same thing