you can use switches or the minimal number of logic gates .
Fuck no, you ain't barring my simple door solution!
I put 6 switches. Three of them close in the existence of water, and the others in it's absence.
Call them A, B, C, D, E, F respectively.
Connect A, D, F in series, B, D, F in series, and C, E, F in series. Combine them all in parallel and complete the circuit with the one-filled-indicator LED.
You can another similar D, E, F switches and connect them in series and connect it to the All three are empty.
The two tank case requires something similar to the one LED thing, only using the reverse instead.
As for a digital circuit: Call the inputs: A, B, C. On if the tanks are empty.
LED1 = OR(A, B, C)
LED2 = [A AND OR(B, C)] OR AND(B, C)
LED3 = AND(A, B, C)