I have 3 white (12) wires coming in to the shed. I have determined which one is the hot but how do I determine which of the remaining 2 is the neutral or ground?
Thank you!

Please don't start a new thread to add additional comments. Whomever installed the 3 white wires was obviously not an electrician. Code would have required color markings. Since you have determined hot, you need to use a continuity or circuit tester to find neutral and ground. The simplest way is to disconnect one of the non hot wires in the panel box and then test for 120 volts between two of the remaining wires. As to your other questions from the first post, with all you want to run, AC, outlets, fans pump etc. 12 is likely not enough for 1 circuit. Do not try to put a larger breaker on that circuit. Have a competent person determine what you need and do the work.

WRT - your question. It is a violation of the NEC to use all white conductors and there are only certain conditions where a white conductor can be re-tasked by permently coloring both ends of the conductor wherever it is visible.

The only correct process you should use is to have a new cable with the proper conductors pulled from the service panelboard to the device box and then make the proper connections.

First, are the conductors feeding a 120 Vac load or a 240 Vac load?

If it is a 120 then you shoud have one Hot, one Neutral and usually a bare ground.

Step one, turn the breaker for this circuit off. Then turn the entire panelboard OFF and remove the panelboiard's top cover. Please be very aware that even with the panelboard main breaker in the OFF position there are still Live connections within the panelboard. The only real way to completley shut the panelboard down would be to pull the meter.

At the branch circuit breaker for this circuit dentify the white conductor on the breaker and either color it "Black" or wrap some black tape on it.

Next, at the fixture using an ohm meter, place one probe fron the meter on one of the white conductors and one probe on the metal grouind of the box holding the device.. If you get a zero "0" reading opn one conductor, then you have fround your equipment grounding conductor, mark this cable with a green coloriung. The other conductor is your White (Neutral) conductor.

If you are sure not how to accomplish the above, call an electrician.