If you have three unknowns, and one of them is a power, i.e 3^{x}, you can use substitution. Make 3^{x}, X. Then make the other 2 unknowns Y and Z. If you are given any of the unknowns, then it is simple. Solve for the others.

If You have three unknowns, and they are, well, unknown, you can again use substitution. For example...

(3^{x}-4y)/6=z becomes (x-4y)/6=z which again, you can substitute which becomes u/6=z

Solve for u, which is u=6z....and so on.

I think the only way to solve for multiple unknowns is to use substitution.

EDIT: actually, making 3^{x} = x (substitution) might be confusing. You can pick any letter, like b or something. But keep in mind what it is. some letters, like s (seconds) or t (time) can get confusing too. It works for any letter, you just have to remember what it is. Always write down somewhere on the paper that 3^{x} = x (or whatever letter you choose)