The synthetic division table is:
$$ \begin{array}{c|rr}3&1&0\\& & \color{black}{3} \\ \hline &\color{blue}{1}&\color{orangered}{3} \end{array} $$The remainder when $ x $ is divided by $ x-3 $ is $ \, \color{red}{ 3 } $.
We can find remainder using synthetic division method.
Step 1 : Write down the coefficients of the dividend into division table. Put the zero from $ x -3 = 0 $ ( $ x = \color{blue}{ 3 } $ ) at the left.
$$ \begin{array}{c|rr}\color{blue}{3}&1&0\\& & \\ \hline && \end{array} $$Step 1 : Bring down the leading coefficient to the bottom row.
$$ \begin{array}{c|rr}3&\color{orangered}{ 1 }&0\\& & \\ \hline &\color{orangered}{1}& \end{array} $$Step 2 : Multiply by the number on the left, and carry the result into the next column: $ \color{blue}{ 3 } \cdot \color{blue}{ 1 } = \color{blue}{ 3 } $.
$$ \begin{array}{c|rr}\color{blue}{3}&1&0\\& & \color{blue}{3} \\ \hline &\color{blue}{1}& \end{array} $$Step 3 : Add down last column: $ \color{orangered}{ 0 } + \color{orangered}{ 3 } = \color{orangered}{ 3 } $
$$ \begin{array}{c|rr}3&1&\color{orangered}{ 0 }\\& & \color{orangered}{3} \\ \hline &\color{blue}{1}&\color{orangered}{3} \end{array} $$Remainder is the last entry in the bottom row $ \left(\color{red}{ 3 }\right) $.