Could someone give me an example of how to do a table of values on a graph when y=3?
The only condition your equation states is that y = 3
That is, there is no condition on x, so the x can be anything.
e.g. (4,3), (1,3), (506,3), (12.34, 3), (π,3), ....
the result is a horizontal line 3 units above the x-axis, that is, the y = 3