My septic system requires 3 circuits. A 10Amp & 2 20Amps. Using schedule 40 conduit in a 80ft run underground to the 3 pole emergency service disconnect. Can these three circuits share a ground? And if so what size does it need to be?
On sharing ground, that's not a problem, it just needs to be sized for the largest circuit. You are vanishingly unlikely to have a ground fault on 2 circuits at once. However:
You're up against a NEC 225.30 limit. You can only run one feeder or circuit to an outside location, unless they are
Different voltages, frequencies, or uses. For example, control of outside lighting...
For instance, you can't run two 120V circuits out there. You can, however, run two 120V circuits if one of them is switched inside the house.
You can also run, separately, a 120V, 120/240V, and 240V-only circuit.
So that gives you a fair variety. Two 120V circuits can be accomplished as a 120/240 multi-wire branch circuit.
And then, of course, each circuit controlled differently is entitled to a circuit - e.g. each section of outside lighting. So if there's any question of putting the well pressure switch at the head vs at the house, put it at the house - it gives you a free circuit.
In fact, hold on -- won't that well pump be 240V*? Of course it will. Your shutoff needs to operate on every hot, so you need 2 blades in your shutoff switch just for the pump. Fortunately, you can have multiple shutoff switches, up to 6.
* By the way, the reason pumps are 240V is voltage drop -- it's not the run from house to well that gets ya, it's the run down the well, which can be hundreds of feet. Well pumps are at the bottom.
Eighty feet is a long run. You will likely encounter some line loss at this distance. I would increase the wire gauge by one greater for each of the circuit's conductors in order to reduce the amount of resistance through the wires and therefore the amount of voltage drop. Watch your conduit fill! Always better to go with a sligtly larger sized conduit in case you have need for more conductors in the future. And larger gauge conductors will also affect your box fill calculations. Spend a bit more money and get the larger sized boxes and larger diameter conduit from the beginning.