Combining variable and number

I’d like to make a script that goes to and plays a random frame (label).
I just can’t remember how to combine a variable and a number.
I have 3 frames called “label1”, “label2” and “label3”
My script looks like this:

_root.frame = Math.floor(Math.random()*3)+1;

But apparently it isn’t right.

How do I combine the text (“label”) with the number (_root.frame) so that it i.e. will goto “label2”?