I have a 60W inverter and it says it needs an input of 12V (DC) @ 6A but yours is only 3A (sounds much more efficient).

Ordinary AA batteries can output 2400mA (2.4A) for an hour each though Energizer claims they can get up to 2700mA (2.7A) for an hour but this is maximum theoretical. Lets just take that down to 2A for an hour per battery for a minimum.

First we need to get up to 12V so 12 / 1.5 (battery maximum voltage) = 8 This means with 8 batteries you can get 12V @ 2A. I will call this 1 set.

For my inverter which is 6A the calculation is 6A / 2A = 3 sets.

For your inverter which is 3000mA (3A) the calculation is 3A / 2A = 1.5 sets.

You cannot have .5 of a set as that would only be 6V so you will have to round up (you can't round down) to 2 sets.

You need 2 sets * 8 batteries (per set) = 16 batteries.

Finally, you can add any extra sets you like to extend usage time.

Wiring. To wire this each set of 8 batteries needs to be wired in series which is negative (-) to positive (+) or positive (+) to negative (-) or just an ordinary 8 battery holder.

Next, to wire 2 sets together you need to wire in parallel which is negative (-) to negative (-) and positive (+) to positive (+).

If this is a car cigarette lighter socket inverter then you will need to wire negative (-) to the outside collar and positive (+) to the center pin.

Obtaining cheap batteries. As you need a minimum of 16 batteries you will want to get them cheaply. I can get 20 AA batteries (PrimePower) from my local £1 shop for £1. You may be able to get it at a $1 or €1 shop etc.

Is it practical? Not really. You'll need so many AA batteries that it doesn't save you any time, money, or space.

Ordinary AA batteries can output 2400mA (2.4A) for an hour each though Energizer claims they can get up to 2700mA (2.7A) for an hour but this is maximum theoretical. Lets just take that down to 2A for an hour per battery for a minimum.

First we need to get up to 12V so 12 / 1.5 (battery maximum voltage) = 8 This means with 8 batteries you can get 12V @ 2A. I will call this 1 set.

For my inverter which is 6A the calculation is 6A / 2A = 3 sets.

For your inverter which is 3000mA (3A) the calculation is 3A / 2A = 1.5 sets.

You cannot have .5 of a set as that would only be 6V so you will have to round up (you can't round down) to 2 sets.

You need 2 sets * 8 batteries (per set) = 16 batteries.

Finally, you can add any extra sets you like to extend usage time.

Wiring.

To wire this each set of 8 batteries needs to be wired in series which is negative (-) to positive (+) or positive (+) to negative (-) or just an ordinary 8 battery holder.

Next, to wire 2 sets together you need to wire in parallel which is negative (-) to negative (-) and positive (+) to positive (+).

If this is a car cigarette lighter socket inverter then you will need to wire negative (-) to the outside collar and positive (+) to the center pin.

Obtaining cheap batteries.

As you need a minimum of 16 batteries you will want to get them cheaply. I can get 20 AA batteries (PrimePower) from my local £1 shop for £1. You may be able to get it at a $1 or €1 shop etc.