Is their a free online calculator to graph linear equations?

How do you graph x=y+3

there are several, try a google search

Usually they need to have the equation in function form, that is y = ....
for yours,
y = x-3

for most linear functions, you really don't need a graphing program, they are really easy to graph.
All you need is two points, in many cases the intercepts are your easiest calculation
in your case, y = x-3
let x=0, then y=-3
let y=0, then x=3
you should be able to visualize the position of these two points, thus the line.