Let A be a square matrix of order 3 then |3A| is equal to

3³|A|

Let  $A=\left[\begin{array}{lll}a_{11} & a_{12} & a_{13} \\ a_{21} & a_{22} & a_{23} \\ a_{31} & a_{32} & a_{33}\end{array}\right]$        $|A|=\left|\begin{array}{lll}a_{11} & a_{12} & a_{13} \\ a_{21} & a_{22} & a_{23} \\ a_{31} & a_{32} & a_{33}\end{array}\right|$

so  $|3 A|=\left[\begin{array}{ccc}3 a_{11} & 3 a_{12} & 3 a_{13} \\ 3 a_{21} & 3 a_{22} & 3 a_{23} \\ 3 a_{31} & 3 a_{32} & 3 a_{33}\end{array}\right]$        $|3 A|=\left|\begin{array}{ccc}3 a_{11} & 3 a_{12} & 3 a_{13} \\ 3 a_{21} & 3 a_{22} & 3 a_{23} \\ 3 a_{31} & 3 a_{32} & 3 a_{33}\end{array}\right|$

⇒  $|3 A|=3 \times 3 \times 3\left|\begin{array}{lll}a_{11} & a_{12} & a_{13} \\ a_{21} & a_{22} & a_{23} \\ a_{31} & a_{32} & a_{33}\end{array}\right|$

Taking out factor of 3 (row by row / column by column)

|3A| = 3³|A|