Then you're in luck! Just go to the Visitor Center the day before, when you check in to pick up your two reserved permits. Ask for one more.
It is only in extremely rare cases that there are no permits available. You see, as people are picking up their permits, they often have members who have dropped out, so they return slots. You will be given one of those.
If you go in to pick up first thing the day before, there may not be any, but as time goes on, there will be, so just stick around and ask repeatedly until you are successful.
At 2 PM is the time they declare no-shows, and there are always additional permits coming available then.
Here are some numbers to look at:
Unused permits, 2015 (Note that these numbers are after everyone looking for a free permit has been given a permit.)
