Find the first three terms in the expansion of \(( 1 + p x ) ^ { \frac { 1 } { 3 } }\) in ascending powers of \(x\).
The expansion of \(( 1 + q x ) ( 1 + p x ) ^ { \frac { 1 } { 3 } }\) is \(1 + x - \frac { 2 } { 9 } x ^ { 2 } + \ldots\).
Find the possible values of the constants \(p\) and \(q\).