feeding 30 amps to detatched shed with 10/3 armored cable?
Armored cable can't defend itself from the rain

You'll need to re-run this cable in order to meet Code.  While you have enough wires there for your proposal as the armor on modern type AC can be used as a grounding conductor as per 320.108, AC cable can't be run in damp or wet locations (such as outside attached to a fence) as that violates 320.12 point 2.  In fact, I would not be surprised if the cable already has water in it, or is even rusting away as we speak, depending on how long it's been out there.

Replacing the cable with a PVC-jacketed (wet location rated) type MC or MCI-A cable is your best bet here as a result.