Can anyone tell me where I can find (or what the part number/name is
of...) the 3-way brass(?) fitting that sits at the rear of the engine
(passenger side of engine bay) and has three cooling hoses connected to
it?

If you follow the two heater hoses coming (from the heater core)
through the firewall, one of them connects to this fitting. The second
hose looks like it comes into the fitting from the side of the engine;
and the third hose comes into the fitting from the front.

1979 280zx
Manual transmission
NON-California model
L28 engine