I'm not sure which one is three off the top of my head, but the stock spark plug wires will have numbers marked on them. As long as they are stock just find number three. You could check first to make sure one of the wires just didn't come loose from the coil pack/plug, but if you have to change them they are relatively easy compared to most cars coming out today. That would be where I would start if everything is hooked up tight, it won't hurt anything and generally not too expensive. It may not hurt to throw fuel injector cleaner or sea foam to clean a little bit as well when you do this. To see how to use seafoam there is a sticky at the top of the list of threads at the "V6 Talk" section.
2001 mustang 6 cyl base model tune up question 
