You want to rendre a black pixel as 1 but the logic in your program does exactly the contrary.
In fact you can simplify the whole thing by replacing
f1=0; f2=0; f3=0;
if (co.R != 255 ) f1=1;
if (co.G != 255 ) f2=1;
if (co.B != 255 ) f3=1;
if (f1 == 1 && f2 == 1 && f3 == 1)
sw.Write("1");
else
sw.Write("0");
with
sw.Write((co == Color.Black) ? "1" : "0");
I seriously question the real interest of the operation, though :)
You should also adopt as early as possible some consistency in the way your are identing your code blocks. And having a look at basic debugging techniques could be quite useful, too.
[Edit]
You never load the image to your _bmp variable.
Maybe try
_bmp = Image.FromFile("MyNumbers.jpg");
to initialize your variable.