\(a+b+c=0\)
\(\Leftrightarrow a+b=-c\)
\(\Leftrightarrow\left(a+b\right)^3=\left(-c\right)^3\)
\(\Leftrightarrow\left(a+b\right)^3+c^3=0\)
\(\Leftrightarrow a^3+b^3+3ab\left(a+b\right)+c^3=0\)
\(\Leftrightarrow a^3+b^3+c^3+3ab.\left(-c\right)=0\)
\(\Leftrightarrow a^3+b^3+c^3-3abc=0\Leftrightarrow a^3+b^3+c^3=3abc\)