I've taken in a stray kitten which I think is about 11-12 weeks old
(it's getting pretty cold here now, and he/she would've been likely to
have frozen to death soon). I'm in the process of worming it right now.
It'll be living indoors from now on, but I was told the other day by a
pharmacist to worm the kitten every three months.

In some cases, a kitten/cat may need multiple doses of a de-wormer to
completely eliminate the parasites, but I have never heard of worming
every three months "just because," especially since this kitty will be
staying indoors.