redirect only every 2nd or 3rd request (no replies)

I'm trying to set up a rule which is going to redirect only every second or third request only if the URL contains a specific string.
I did already the redirect based on a string in the URL but I can't find how to redirect only every second on third request:

if ($request_uri ~ .*.WHATEVER_STRING.*) {
if (this is the 2nd request than) {
rewrite ^/(.*) WHATEVER_URL;
}
}