a=999x+1000,b=999x+1001,c=999x+1002
∴a-b=-1
b-c=-1
c-a=2
a²+b²+c²-ab-bc-ca
=(a²-2ab+b²+b²-2bc+c²+c²-2ca+a²)/2
=[(a-b)²+(b-c)²+(c-a)²]/2
=[(-1)²+(-1)²+2²]/2
=3