For the following reactions at a certain temperature:

(i) $\text{H}_3\text{PO}_4(aq) \rightleftharpoons \text{H}^+(aq) + \text{H}_2\text{PO}_4^-(aq)$; $K_1$

(ii) $\text{H}_2\text{PO}_4^-(aq) \rightleftharpoons \text{H}^+(aq) + \text{HPO}_4^{2-}(aq)$; $K_2$

(iii) $\text{HPO}_4^{2-}(aq) \rightleftharpoons \text{H}^+(aq) + \text{PO}_4^{3-}(aq)$; $K_3$

The equilibrium constant for the reaction $\text{H}_3\text{PO}_4(aq) \rightleftharpoons 3\text{H}^+(aq) + \text{PO}_4^{3-}(aq)$ in terms of $K_1$, $K_2$ and $K_3$ is