One plus negative one is zero.
No. Zero is neither plus nor minus.
It's one !
"minus zero" is the same as plus zero in that it makes no difference to the sum. So the answer is -3.
Perhaps you mean minus minus one is plus one. Like subtracting (taking away) a debt is equivalent to adding on. If you bank balance is two dollars in the red (debt) if you add 2 dollars, that debt has gone. minus 2 minus minus 2 is zero, same as minus 2 plus 2 is zero.
It can be a plus or a miuns - depending on which one of the two had a greater absolute value. Also, it will be nearer to zero than the larger of the two magnitudes.