The fact that you switched coil packs around and the problem decreased is a hint. Did you simply switch packs around, or did you exchange some of the packs with new(er) or known good ones?
Has the car been stored in a garage or outside? 17k mi is very low for coil packs to fail unless moisture or temperature are factors. Look carefully at the top of each plug for signs of corrosion (usually green in color). Do you have radio noise (popping in sync with the engine speed)at idle? Just pulling the coil pack on & off can disturb corrosion enough to make things work a little better.
Try this for cylinder 3, if its still the only one with a problem. Pull the coil pack and look at the top of the spark plug (the electrical contact with the coil pack) for any signs of green. Clean the contact with sandpaper, copper pot cleaner, etc -whatever is handy. Then, take a pencil with an eraser on it, put the eraser down into the coil pack rubber tube and rotate it in the pack's electrical contact. Reinstall the pack. If the problem is gone, its time to replace the coil packs as well as the spark plugs.
As far as the health of that rubber insulator tube on the coil pack, Larry B's trick is to take the plug and coil pack in hand, insert the plug into the pack. The plug should not fall out of the tube. (Did I get that right Larry?)