How do you graph y equals -3?

Y = -3

The graph is a horizontal line, with zero slope, passing through the point Y=-3 on the Y-axis. It also passes through the point (any X, -3).

-- Place a pencil dot at the point -3 on the Y-axis.

-- Go anywhere else on the paper that you want to, right or left of the Y-axis. Mark another point at 3 units below the X-axis.

-- Connect the two points with your ruler, and continue the line as far as you want, in either or both directions.