Sequence Command
Define this and try to keep it unsimplified. (Check the toolbar for that)
f(x) := (x^2 - 9)/(x - 3)
Now use the command sequence to perform several substitutions at once. (Click here to know more about sequence)
Sequence[Substitute[f(x), x = a], a, 1, 5]
Try the same with:
Substitute[Simplify[f], x = 3]