I have recently added a second panel box in my house for my garage.. And I have a box in the garage... My question is that I'm wanting to add a 20amp breaker to my garage (swimming pool) do I need to shut off power to all 3 boxes? The 1 feeding my garage or just my garage box?

Cut the main breaker off right above the breaker you want to add. The main thing becarefull don't touch anything because the main wire right where you working are still hot.

I'm just wanting to add a plug to my garage panel box... So do I shut off power to that box or the boxes in my house?

Should be safe just shutting off the breaker that serves the panel that you want to add a breaker to.

Should be safe just shutting off the breaker that serves the panel that you want to add a breaker to.
Just turn off the panel you working on but the main feed wires are still hot. What you doing is very easy just be careful .

You are adding a circuit for a pool. There are a lot of codes involved with this. Are you familiar with them all?

Just turn off the panel you working on but the main feed wires are still hot. What you doing is very easy just be careful .If this is a sub-panel, and the feeder breaker in the main panel is turned off, the lugs will not be hot and the whole panel will be dead.

If this is a sub-panel, and the feeder breaker in the main panel is turned off, the lugs will not be hot and the whole panel will be dead.some reason they don't want to cut off main panel it don't matter if they don't but I'm not understanding why...

some reason they don't want to cut off main panel it don't matter if they don't but I'm not understanding why....
He is right about code comment. Really an electrician should be called out. So be extra carefully when doing this yourself since you did not study this field of work.