$\sum_{i}^{n}\sum_{j}^{i}\sum_{k}^{n} O\left ( 1 \right )$
$=\sum_{i}^{n}\sum_{j}^{i}O\left ( n\right )$
$=\sum_{i}^{n}T\left ( i \right )O\left ( n\right )$
$=\left ( 1+2+3....+n \right )O\left ( n\right )$
$=O\left ( n^{2} \right )O\left ( n\right )$
$=O\left ( n^{3} \right )$