Generating, Evaluating, then Re-generating Random Numbers in Ruby

Noob here. Thanks in advance for any help, and I hope this is articulate enough to make the question clear.

I need to generate random values for a few variables. And then, if those values meet certain characteristics, I need to trash them and re-generate (re-define the value of the variable?).

I probably need to do this with a function but I'm just learning those and not sure how to escape the "locality" of variables created in a function (I'll need to call them later in the script).

So let's say I have:

x = rand(1...9)
y = rand(1...9)
z = rand(1...9)

Afterward I need something like

if x - y - z < 0
then **start over** until x - y - z > 0
end

It seems like this must have a "simple" solution that I just don't have the experience/knowledge to see. Any ideas appreciated!