If I may add a few tidbits of information, forget the injectors. Chrysler buys their injectors from Bosch in flow-matched sets, and has extremely little trouble with them. Since 1990, I've read about one bad one. If you were to suspect a bad injector or spark plug, swap that one with one from a different cylinder. Erase the fault code, then see if a code sets for the cylinder you moved the suspect part to.
When you have a running problem, you need to list the engine size right away. There can be four or five, and some of them have wildly different ignition systems from the others. You also need to list the mileage and transmission type. We make generalizations based on that when deciding the order of things to check. It's not fair to make us guess, or to waste time dragging that top secret information out.
You're right that number 3 is the rear, center cylinder. It is easy to get to from underneath when the van is on a hoist. If you try to do it from the top, the long black bucket holding the wiper system can be removed to improve access.
The 3.8L uses three ignition coils in a coil pack, or assembly. The easiest is to get a good used one from a salvage yard to try as a test. While it's not real common, a partially-shorted coil can fail to develop sufficient voltage to fire the pair of spark plugs under all conditions. What you hear as a backfire could be the unburned fuel from a misfire going into the exhaust system where it fires and creates the sound of a backfire.
A deteriorated spark plug wire can cause the same symptom as a weak coil. A better suspect is a failing crankshaft position sensor or one that was installed improperly without the spacer needed to set its critical air gap. If the history of that sensor is unknown, your mechanic can use a scanner with graphing capabilities to record the sensor's signal while the problem occurs. If you see the signal pulses drop out completely, intermittently, suspect the sensor is failing. If you see the signal voltage drop just a little, but it's enough for the computer to be unable to read it, resulting in a momentary misfire, suspect the air gap is too big.
You don't have to run to your mechanic to have diagnostic fault codes read. Chrysler makes doing that yourself much easier than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds, without cranking the engine, leave it in "run", then watch the code numbers appear in the odometer display. You may only get a code indicating which cylinder is misfiring, but if you're lucky, one will set related to the crankshaft position sensor. Remember that fault codes never say to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a part is referenced in a fault code, it is actually the cause of that code about half of the time. First we rule out wiring and connector terminal problems, and mechanical problems associated with that system.
Also be aware a lot of people, including some mechanics, still believe there is no fault code to be read if the Check Engine light isn't on. That is absolutely not true. There can be over 2,000 potential problems detected. About half of them refer to something that could adversely affect emissions. Those are the codes that turn on the Check Engine light. A fault code related to a crankshaft position sensor or camshaft position sensor will usually not turn on the Check Engine light. That is because with a totally-failed sensor, the engine won't run, and a non-running engine can't create excessive emissions.
Wednesday, February 22nd, 2017 AT 3:27 PM