Starts then shuts off

I charged it to 12.5 then turned it on and immediately started dropping down and shut off.

When it was off it was 12.5 but as soon as it turns on it drops to around 10.5 and just keeps going down.

It's a brand new battery. You think it's bad already? It's not even 3 days old.

Once I turn it off the car, the battery stays at around goes up to about 11.8 and stays there.

Sorry, I have been busy so I hadn't had time to work on the car. Okay, earlier this week I looked at it more closely. My dad unplugged the alternator wire from the alternator but left it connected to the battery on a 200amp fuse which has two terminals. I then unplugged it from the battery so that was eliminated completely. I then hooked up another wire from the alternator to the battery at the opposite terminal at the 200 amp fuse connection and it seemed to be working for about 2 days, the only thing is it wouldn't start up in the morning until about the 3rd push of the ignition button. So we got a new fuse to replace the old one and hooked everything back up like normal and it went back to the starting up and immediately stalled.

It turns off within about 2-3 seconds.

Everything comes on on the dash like normally. No check engine light or anything just the battery light stays on again. But I've been checking it and it stays at around 12.6v.

Yes, the red security light comes on and then goes away when the engine shuts off.

Okay. The security light comes on and goes away within 2-3 seconds.

Also when I do that without turning on the car, it gives me a message on the radio screen to enter anti theft code, but only when I push the "start" button on the car without starting the engine.

I can keep starting it up and it keeps stalling.