I have 3 2.5mm^2 T+E cables going to the garage, in a duct.
They are about 30m long.

Assuming 16 ohms per Km, that's .55 ohms.
Or around 40A, on voltage drop alone, so that's not a concern.

Why 2.5mm^2?
I had planned to put in one 2.5mm^2, but I realised as I was about to
drag it through the conduit, that I had all this spare cable...

The duct is buried in the ground, and is 36mm in diameter.

The easy approach is to put a 40A fuse at the close end, and parallel
all the wires.
Of course, this isn't very safe, as there are unfortunately joins in the
cable.
My current plan is to common all the L/N/E, put a 13A fuse on each
live, and at the garage end, have a RCD CU, with a seperate earth spike.

Anything obvious that I've missed?