It is known that every number can be represented by a sum of $3$ triangular numbers. According to Gauss (see formula $35$ in mathworld article) $$ \text{num}=\Delta+\Delta+\Delta $$ I did some numerical experiments that suggest the above formula is correct when triangular numbers are replaced by tetrahedral numbers $$ \Delta=\frac{n(n+1)(n+2)}6 $$ if $n$ is allowed to be negative.
Is this conjecture correct?
I tried to google representation of integers by tetrahedral numbers but didn't find anything.