Does it take/cost power when supplying in 3v to a "5v to 3v" regulator
Attempting to pass through 3V through the 5V to 3V regulator may result in erratic behavior by the regulator as it attempts to regulate below its drop out voltage, and possibly a lower voltage at the output. The regulator will also take some power for itself. It's not a straight forward option. You are better off bypassing the regulator.