Sometimes (although rarely) i need to shut down my server completely and turn it on again, when outside of the house.
Is it possible to wire the Pi3 to power switch pins on the motherboard?
The power switch pins just need to be shorted/jumped for the PC to be turned on
Yes just use a 3V reed relay or an optocoupler to isolate the +3V/0V output of the Pi from the switch jumper.
I think I know what meaner awesome, thank you.
And the 3v output can be controlled via HA?
Yes. GPIO Switch:
You should create a script that turns the switch on, waits a short delay and then turns the switch off. And use this from the from end (don’t show the switch) or automations. You know what happens when the switch is held on - forced reboot.