Fill in the gap to factorise this expression. \[ 3 x+3=3\left(x+\_\right) \]

Step 1 ：Divide both terms on the left side of the equation by 3: \(\frac{3x}{3} = x\) and \(\frac{3}{3} = 1\)

Step 2 ：Insert the simplified terms into the expression: \(3(x+1)\)

Step 3 ：\(\boxed{3(x+1)}\) is the factorised expression.