Old 3 handle bath tub faucet replacement

[PSP]

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 fitting.
Please correct me and provide me your valuable tips.
Thank you!

 

[Snoonyb]

Welcome.
#1 is a union, which is typical for a faucet connection, and the new faucet should have a "complete union", as well.

Channel locks and some WD will work for the removal, except you'll need something to cut the shower arm riser. IE, tubing cutter or sawzal.

Whoever assembled the faucet and tub spout simply used the metal pipes, handy. I always used brass.

 

[PSP]

Thank you, Snoonyb.
The new valve doesn't come with union (Pfister bedford 3 handle).
I hope I can buy the union as this would be necessary. Alternatively,
instead of the union for the hot and cold, could I use 1/2" copper female
threaded pipe fittings, extend with 1/2 inch coupling fittings?

The old valve has female thread for the bath tub and shower connections,
while the new valve have male connections.
Thanks!

 

[Snoonyb]

I would use brass unions for the supply and a shark-bite coupling for the shower, a brass coupling, nipple, elbow and nipple for the spout. Saves having to disassemble, or heat sink the faucet to prevent heat damage from soldering.

 

[PSP]

Thank you!