a^2+b^2+c^2-ab-bc-ac
=(1/2)*(a^2+b^2-2ab+a^2+c^2-2ac+b^2+c^2-2bc)
=(1/2)[(a-b)^2+(a-c)^2+(b-c)^2]
=(1/2)[1^2+1^2+2^2]
=3