I'm a beginner in electronics. Currently trying to set up 3 cooling fans (DC 12V 0.1A) in my 3D printer.
If I were to connect all 3 of them in parallel straight into 12V 30A power supply unit, will it overload the cooling fans since its working amp is at 0.1A only? or does the PSU regulates the amp's output for whatever the working amps are?
No it wont overload it, this will work perfectly fine. As long as the voltage is correct, and the power supply can provide as much or more current than the device or devices need, then you are good. The current rating of that power supply is simply telling you the maximum it delivers, the actual amount it delivers will be determined by the load's resistance (in this case the fans).
So yes any DC power supply that provides 0.3A (0.1A x 3) or more power will work for you.
While there are many different kinds of power supplies, and supply/load interactions, yours is the most common.
The supply is a constant-voltage type. It tries to impress 12 V across whatever is connected to it. But the current is up to the load. Load current is drawn from the supply by the characteristics of the load. So while many switching type supplies have a minimum load requirement, at the most basic level the supply can power 1 of your fans or 50; all of the fans will see 12 V, and each will draw the current it needs.
