Redirect only every 2nd or 3rd request

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;
}
}

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