Is a constant or a variable?
Hopefully others can supplement the following with more precise language...
3^(something) could be interpreted in a few ways, depending on what that "something" is.
If "something" is a number, then it's obvious: 3^2 = 3^2(!) = 9.
If "something" is, say, a/b/c/d, then we might interpret a/b/c/d to be a constant, in which case 3^a, while still "depending" on the value of a, doesn't vary once we have defined a.
If "something" is, say, u/x/y/z, then - as everyone has to this point - we interpret these letters are being variable.
See http://en.wikipedia.org/wiki/Variable_%28mathematics%29 (under heading "Notation").
Yeah, I know. It's wikipedia. But this way, we're all pulling from the same definition!