You do not have any other PSU ?

 

No and that one was brand new so i would like to use what i have if possible.

Is it possible to use 2 different 12V 3A power sources? Thanks

 

It is possible.
You have to use low Vf high current diodes in series with each PSU to protect the PSU's from dumping current into each other when the voltage mismatches.

 

Okay. I don't have diodes like that and that sounds complicated.

In one video some one used some sort of car battery charger.

I could buy product like this if i'm sure it works and outputs enough juice all the time: box says 12V / 4,2A but box has also big 6A and spec says: 6A 12V 230V ?-45Ah. What’s the difference between 6A and 4,2A. Refrig needs 6A.

 

If the box needs 6A then you need minimum 6Amps. A 4.2A one will be in sufficient and will go into protection if you try to draw more than 4.2A.
I suggest you find something that can supply more than 6A. More is advisable. Supply will run cooler.

 

A 12V 6A battery won't last long although it can run the fridge till battery discharges.

 

Is there any other cheap power supply which outputs 12V 6A and i could use other than these tower computer type of power supplies? Thanks

 

Ya...
Laptop PSU's.
But that needs to be modified to reduce output or u need to build a PWM drive for the fridge.

PC PSU is the cheap option