NodeMCU power supply?

Hi guys i need help. So i have always used arduino for projects but i always got it plugged in my laptop so i never have to worry with my power supply. But now i need to power my project without it and i wanted to use my old computers power supply it has a bunch of outputs and im thinking of using the 5V 3A to power my vin nodeMCU and my Arduino Mega. Now what i want to know is if this is ok will it not cause any damage to the boards and will i still be able to use the 5V and 3.3V in the boards output pins to supply some of the Sensors? Thanks

A regulated 5v supply will drive the NodeMCU correctly provided it is connected to the 5v input so that the on board 3v3 regulator can be powered. Since the NodeMCU can pull about 1 Amp when it's busy, the 5v 3A supply will be fine.

For the Mega, the regulated 5v supply would not drive the on-board voltage regulator to 5v, since the regulator causes up to 2v drop. In other words you'll get 3v or similar and the board will likely not work properly. You need to provide at least 7v (7-12v recommended) to properly power the board. If you connect the regulated 5v to the USB port power connections, you could use it directly as the USB input bypasses the 5v regulator.

I have a few standalone NodeMCU boards and use old USB phone chargers to power them. That is, USB "wall wart and a USB cable to the NodeMCU input. Pretty much any phone charger rated for 0.5 A output or greater should be fine.

The 5V/3A output from your computer power supple ought to be ok, but seems like vast overkill for powering a single NodeMCU. Also some large capacity power supplies specify a minimum current draw to maintain the specified voltage regulation.