How do you find the slope and intercept to graph #y=3#?
This is a straight line parallel to the x-axis.
See explanation for actual intercept.
You asked "how is..."
When stated like this question, it is saying that every single value of y is 3. That means the plotted line is not permitted to get any 'higher' or 'lower'. Slope (gradient) is the change in 'up' or 'down' for a given change in x. There is no change in 'up' or down' as it is parallel to the x-axis. So the slope (gradient) is zero
Consequently the only axis it crosses is the y-axis. So the intercept is when x=0 and y=3: