Old wiring coming in to rebuilt structure has three white wires (12), none marked or color coded. How can I determine which one is the hot, neutral and ground. Also wiring is for outlets, lights, LARGE exhaust fan, window a/c unit with a pond pump on the same circuit. Is Romex 12/2 w/ ground OK to use?
Thank you!

Is the wire marked THHN/THWN?
After you figure the whites, you're running a 12-2 as well?
The hot is easy, it the neutral and ground to be more challengng. Plug an extension cord into a known good outlet. Place one lead into the cords ground, then look for voltage on the three, identify with black tape. Identify that circuit/Breaker. Turn Off all breakers but this one. Follow that wire where it leaves the panel, should go into a pipe(conduit), you may have just the 3 whites(which is wrong), or other circuits as well. Follow from that conduit and see what other circuits leave the panel in that conduit, may help trace wiring back.(Say the Washer goes out the same conduit, then the change to whites may be there.

With just that one circuit on, remove the ground or neutral at the Panel, and see which is lost on the remaining 2 whites, Identify.

The Neutral carries current, The Ground keeps everything at Ground potential, to keep equipment from becoming Live.
Good Luck