$3\mid a^3+b^3+c^3$ if only if $3\mid a+b+c $ 
Prove the following equivalence: $3\mid (a^3 + b^3 + c^3) $ if and only if $3\mid (a + b + c) $.

My try:
I know $a^3 + b^3 + c^3 = (a + b + c) (a^2 + b^2 + c^2 – ab – bc – ca) + 3abc$, but I can't seem to proceed from here.
Thanks all!

Compare $a^2+b^2+c^2-ab-bc-ca$ with $(a+b+c)^2$
