More than 1 esp8266 on 5v buck causes flickering

My plan was to use one 5v 2A buck converter on my 12v PSU to power 3 ESP8266, if I do this I get severe flickering if more than one is in use.
If I use 3 buck converters then all is fine.
Does anyone have an idea on how to stop this ?

I want to put 6 ESP8266 ideally in one box so having 6 bucks is not going to fit easily.

Need to see your wiring diagram… also are you using nodemcu? If so vin take 12v

The wiring is the typical method as In image except the 5v powers 3 Nodemcu.
My nodemcu specifies 10v as a max on Vin. The buck convertor has a common 0v rail.

If I just power 1 nodemcu it all works perfectly, if I power all 3 they all get flicker. If I use a separate buck converter for each node mcc it also works ok.

I even tried using a regulated 5V psu to power the nodemcu’s and joined the 5v and 12v 0v rails with the same issue

Maybe the converter is isolated. Try connecting the grounds.

It has a common ground, I have checked it.
Everything works fine with just 1 nodemcu. The problem occurs when I power 2 or more from the same
buck converter. It is very odd.