If the magnitude of the vectors, \( \overline {a} , \overline{b} , \overline{c} \) are 3, 4, 5 respectively and \( \overline{a} \) and \( \overline{b} + \overline{c}, \overline{b} \) and \( ( \overline{c} + \overline{a} ), \overline{c}\: and\: ( \overline{a} + \overline{b} )\) are mutually perpendicular, then find the magnitude of \(( \overline{a} + \overline{b} + \overline{c} ) \)