@Antoniopty 

Hi, for your issue, we would like to confirm the below information.

 'After a while, 2 of them stopped working. They only show the first two leds and then turn on all the leds and powers off. This behavior repeats over and over.'

As your description, the status is first two green lights, then all the lights are green suddenly, finally all the lights are off, right?

Would you like to send us the picture of the status to check?

Can you also check the hardware version and firmware version of the powerline device?

If it is not the latest one, please follow the instruction to upgrade the firmware version of the two extenders to the latest one.

https://www.tp-link.com/support/faq/1419/

Note: Please connect the PC to the extender by cable to upgrade its firmware version.

After upgrading the firmware version, please check the lights' status again.

May I know if there is sudden power outage or storm in your house?

The three extenders are bought one by one, right? May I know when and where you bought the extenders?

Have you saved the purchase proof?

Look forward to your reply.

Have a nice day!