Please Help! P0320

P0171 is system lean bank 1.
P0303 is a misfire on cylinder 3.

Those could be related since cylinder 3 is on bank one.

The easiest way to diagnose the misfire is the following:
1. Swap the spark plug from cylinder 3 to cylinder 2 and vice-versa, start the engine, and check for a cylinder 2 misfire. If there is one, change all the plugs.
2. If the misfire doesn't move to cylinder 2, switch the plug wires (at both ends) for cylinder 3 and another cylinder that's long enough to reach it. If the misfire moves to the other cylinder, get a new set of plug wires.
3. If the misfire doesn't move to the other cylinder, you can take the ignition coil off and take it to most parts stores to be tested. If it fails the test, replace it, if it passes, there is one more check of the ignition system.
4. If the coil passes the electronic test, it could still be bad on the secondary side. http://easyautodiagnostics.com/ford/3.0L-3.8L/how-to-test-the-coil-pack-1 has a good guide on how to check it.

If all of that fails to isolate the cause of your cylinder 3 misfire, you either have an injector that's not squirting fuel, or a lack of compression in that cylinder. Low compression doesn't usually result in a lean code, so at that point I'd strongly suspect the injector not spraying fuel into the cylinder. You can swap the injector to another cylinder like the spark plug in the ignition check, but it's a bit of a pain and can be messy. You can also get a set of noid lights to make sure the injector is getting signal from the PCM. If it's not getting signal, the problem is in the wiring or the PCM itself. If it is, the injector is bad or you have low compression.

Overall I'd suspect an injector-related failure because of the two codes present, but I'd check the ignition side first because it's easier and to be sure.