\[3x^{2} - 12x + 3 =\]
\[= 3 \cdot \left( x^{2} - 4x + 1 \right) =\]
\[= 3 \cdot \left( x^{2} - 4x + 4 - 3 \right) =\]
\[= 3 \cdot \left( (x - 2)^{2} - 3 \right) =\]
\[= 3 \cdot \left( x - 2 - \sqrt{3} \right)\left( x - 2 + \sqrt{3} \right)\]