Hello, I was wondering where I could find a relay for relay spot #3 on the interior fuse panel

-Thanks

 

That position is not used unless you have a model that does.
To use it you will need to see if power is present and if you can get leads from the underside for your application.
That may require putting connectors with lead in the slots.
Then you need to operate the relay with a control switch and supply power to the circuit. There will be 4 different leads involved.
Make provision for fusing the circuit.
As far as obtaining a relay, visit Ford parts dealer or get one from a yard.
If this is not what your doing, disregard the above.
Good luck.