(a-b-c)^3
=(a^2-ab-ac-ab+b^2+bc-ac+bc+c^2)*(a-b-c)
=(a^2+b^2+c^2-2ab-2ac+2bc)*(a-b-c)
=a^3-(a^2)b-(a^2)c+a(b^2)-b^3-(b^2)c+(a^2)c-b(c^2)-c^3-2(a^2)b+2a(b^2)+2abc-2(a^2)c+2abc-2b(c^2)+2abc-2(b^2)c-2b(c^2)
=a^3+b^3+c^3-3(a^2)b+3a(b^2)+3a(c^2)-3(b^2)c-3b(c^2)+6abc