This is an example of nim. I would recommend Conway's On Numbers And Games (ISBN: 1568811276) if you want a terrific read on the subject and much, much more.
I'll present the typical strategy, though I won't prove it (and I would post this as a comment, but I'm afraid I'm not allowed...):
Suppose we have a stack of $3$, one of $6$ and one of $7$. We now wish to find the value of this position. We do this by first converting the sizes of the stacks to binary ($011, 110$ and $111$), and then add them without carrying:
So the value of our position is $2$. Now, to find a winning move, we add $2$ to one stack at a time, in the same manner as before. In doing so we get $001, 100$ and $101$, respectively. Pick any one of those alternatives that are legal (meaning they reduce the size of the given stack. In this case all of them are) and you'll be giving your opponent a position of value $0$, meaning it's a losing position.
For example, $001, 110$ and $111$, or $1, 6$ and $7$: