The position vectors of points A and B relative to an origin O are given by

\(\overrightarrow{OA} = \begin{pmatrix} p \\ 1 \\ 1 \end{pmatrix}\) and \(\overrightarrow{OB} = \begin{pmatrix} 4 \\ 2 \\ p \end{pmatrix}\),

where \(p\) is a constant.

1. In the case where OAB is a straight line, state the value of \(p\) and find the unit vector in the direction of \(\overrightarrow{OA}\). [3]
2. In the case where OA is perpendicular to AB, find the possible values of \(p\). [5]
3. In the case where \(p = 3\), the point C is such that OABC is a parallelogram. Find the position vector of C. [2]