Shared ground for 3 circuits
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.