Old 3 handle bath tub faucet replacement

I wanted to change my old 3 handle bathtub faucet and valve with a newer one.
I wanted to make sure I know the connectors used, before trying to
dismantle the old valve system. I will be referring to the notations
in the attached image:

What is "1" fitting called? What are the best ways to
dismantle a connection with "1", "2" and the 1/2 in. GI nipple (3) below.
(The GI pipe below "3" has corrosion and I plan to replace it).
Just below the GI pipe is a 1/2 in. copper FIP female adapter...

Old 3 handle bath tub faucet replacement