Probably need new plugs, and ignition coil, and depending on spark plug wires maybe them. Approximately 200 for parts. Anyone with tools and a little knowledge can change them out in an hour.
Depending on make, model, mileage etc, this could be caused by the aforementioned items, but could also be a timing belt issue (again depending on make, model, engine, miles), a distributor cap (if so equiped), an ignition coil, and lots of other things. Finding the exact cause will require some diagnostic tolls, some practical expertise, and a bit of time.
Cars, like people need to be diagnoised and repaired in person!!!
Sounds like she already had code reader hooked to computer to get the # three misfire, which probably means no distributor cap. So other than timing belt which usually are good for 75-90,000 miles. The other parts would almost be a complete tune-up. If you pull # 3 plug and find oil all bets are off.
*Fowled Plugs...caused by carbon build up in cars, too much alcohol in humans...
*Bad Gas...caused by water in cars, burritos in humans...
*Clogged Air Intake...dirty filter in cars, cock in mouth in humans...
I had one this weekend that showed "multi cylinder misfire" on the diag tool and it did have a distrubutor cap.
Almost all of the answers in the thread could be the cause, but there is no way to know for sure without some hands on diagnosis. I sure do miss the days when cars and truck engines were simple!!