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I'm questioning whether connecting multiple GPIO pins (which to my knowledge are 3 volts), to one ground pin, is going to damage my Raspberry?
I want to connect GPIO pins to breadboard, pass the current through a resistor and a LED, and then pass the current into one GPIO ground pin for about 3 GPIO pins.
Setting aside the ill-phrased part of the question (connecting multiple GPIO pins (which to my knowledge are 3 volts), to one ground pin) - which indeed short circuits the GPIO pins if they are set to output and thus most likely damages the Pi, there is nothing wrong with using one GND pin on the GPIO header for multiple GPIO pins. As you notice there are not as many GND pins as GPIO pins.
The GND pins are connected to a common ground plane on the printed circuit board of the Raspberry Pi. The current that returns to the Pi is not flowing through the sensitive microelectronics (the processor). It is simply limited by the wiring and the header connector - which are safe to return the current of three GPIO pins (a few 10 mA in total).
If you were looking for an answer to the simpler question:
Can I connect a GPIO pin, directly to a ground pin?
The answer is:
Only if you remember not to set that pin as output.
You should keep it as input, otherwise you will probably destroy the specific pin when you try to write a high value to it.
