C B A->Y
0 0 0 Y=1 
1 0 0 Y=0 
1 1 0 Y=0 
0 1 0 Y=0 
0 1 1 Y=0 
1 1 1 Y=0 
1 0 1 Y=0 
0 0 1 Y=0 
function: Y = (!C && !B && !A)