First point--how are you getting to the terminal? If you are using
DCL transportation, your terminal arrival time (scheduled) really doesn't matter. If you are taking DCL transportation from MCO, you will actually check in for the cruise at the welcome center at MCO and will get a boarding card as you board the bus. You will get a better number than you would have at the terminal! If you are taking DCL transportation from WDW, you will be allowed to check in and board when you arrive on the bus.
OK, if you are using private transportation--you can arrive at the terminal early if you wish. However, your boarding card number will be based on your scheduled arrival time (it will do you no good to arrive early to try for a better boarding number). If you arrive early, you may be required to wait to enter the terminal until some of the crowd had been transferred to the ship as the terminal capacity cannot be allowed to exceed the fire regulations. Boarding starts at 11:45, so any time after that, you can normally get into the terminal without difficulty.
You will check in and get your pre-assigned boarding number. There is every chance that Palo will be available on a longer cruise. On a 3 night cruise....good luck! Check the Welcome Aboard navigator that you should get at check in for the times and location of Palo Ressies. On the Wonder, they didn't start till 1 pm and wouldn't allow guests to line up in advance! On the Magic, it was open upon boarding at 11:45. They are supposedly working to standardize things across the 4 ships, but who knows? Pay attention to the Navigator.