Plot the original graph of f(x), then shift it upward by 3 units to graph g(x)=f(x)+3.
If you have the graph of the linear function f(x), to graph the function g(x)=f(x)+3, you would simply shift the entire graph of f(x) upwards by 3 units.
Let's say the graph of f(x) is a straight line on the coordinate grid. To graph g(x)=f(x)+3:
Plot the original graph of f(x).
Then, for every point on the original graph, move it up by 3 units to get the new points for g(x).
For instance, if f(x) has points like (1, 2), (2, 4), (3, 6), etc., then for g(x)=f(x)+3, the corresponding points would be (1, 5), (2, 7), (3, 9), etc.
This shift of +3 units upward applies to every point on the original graph of f(x), so your new graph g(x) will be the same as the graph of f(x) but shifted upwards by 3 units.