Ford 3.0l is the same FF or not.
FF vehicles have a fuel sensor and larger injectors, and of course a computer than can use them.
So #3 is the passenger side rear cylinder
Yes, spark plug is a good place to start.
Before pulling it out I would unhook coil, for a no start, crank engine, then hook up the coil and pull out #3 spark plug.
Make sure spark plug is wet with fuel, that will tell you injector for #3 is working, so two birds with one wrench