Code:
a,b,c
d = a-b;
get MSB of d
if(MSB)
  no is +ve and a>b and so compare a and c
  //Number d is positive and so we get a as greater than b 
  //Now compare a and c in similar fashion
else
  no is -ve and a<b so compare b and c in similar fashion.
  //Number d is negative and so we get a as less than b 
  //Now compare b and c in similar fashion
I hope this comments as explanation helps you to understand.

Thanks
Shabbir Bhimani