I don't think it's even possible to have an entire FFS procedure done in less than 3 hours, depending on what you factor into the "entire" procedure. Most girls I know who have had a large amount of FFS have had at least 10 hours, if not more. As an example, I split mine into two surgeries and they both took between 4-5 hours from what I remember.
Why are you assuming that the the anesthesia will only last 3 hours? They will put you under for however long they need to complete the surgery.