How do i graph this step function? f(x)=[[3x]]+3

One way is shown here: https://www.desmos.com/calculator/76qirhx3nh
___
A step function is defined various ways. One way has [[0]] = 1/2. That is what is shown on the graph at the above link. Choose the definition that matches your text.

The horizontal scale factor of 3 (in 3x) does nothing. When x > 0, 3x > 0; when x < 0, 3x < 0. The +3 adds a vertical offset to the graph of [[x]].