I hope the 3v you are applying with the switch is through a resistor. If not, then you are basically shorting the 3v to ground in your port when you have previously written a zero to that pin. Without a resistor to limit the current you are very likely going to damage the pin driver circuit.
From your statement that the input works the same with a 1 or 0 written to the port seems to indicate that your 3v was overcoming the 0 output of the pin. If you had a resistor in series with the 3v to the pin then the pin would remain at 0, with the resistor absorbing the power. Without the resistor, the port drive circuit would be sinking excessive current.

Hal