I'm interested in setting up a 3-month sustainer option — one gift each month for October, November and December. Is it possible to configure CiviCRM to do this? Or, if that's not possible, could I configure it to set donors up as monthly givers, but automatically cancel after the third gift?
Absolutely - you can configure things such that the recurring series will be for 3 instalments only. That way you don't need to manually cancel them after the third gift.