$(-\infty, 3]$ Refer to the image below for the graph.
Work Step by Step
The solution set of the given inequality includes all the numbers that are less than or equal to $3$. The set includes all the numbers to the left of $3$ so $-\infty$ will be used on the left side of the interval notation. The highest number is $3$ so a bracket will be used on the right side of the interval. Thus, the interval notation equivalent of the given inequality is: $(-\infty, 3]$ To graph the solution set on a number line, plot a solid dot at $x=3$ then shade the region to its left. (refer to the attached image in the answer part above)