The cylinders on an engine are numbered in an arbitrary order. Cylinder # 3 is somewhere in the center of the engine. This code means that the fuel in that cylinder is not being burned consistently. The most likely cause is a defective spark plug or plug wire. So the simplest approach would be to replace the spark plugs in all the cylinders (since the plugs all usually wear at a similar rate; and the other plugs may also be on the verge of failing). If replacing the plugs cures the misfire; the cost should be the price of a set of plugs; plus the labor to install them. This should cost between $30 and $100, depending on the type of engine you have, how many cylinders it has, and how much the spark plugs cost. You can get a general idea of the charge by phoning a few shops in your area, telling them the make, year, and model of your car, as well as how many cylinders it has (if you know), and asking the cost of spark plug replacement.
But, without further testing, it cannot be accurately determined whether just changing the plugs will fix the problem; or whether additional work will be necessary. Sometimes a cylinder will misfire because of worn or defective internal parts in the engine. And that could cost a large amount to fix. Other times a cylinder will misfire because of a leak in a vacuum hose, or a sticking EGR valve. But it is most often just caused by a bad spark plug.
If you want to get a definite idea of the cause of the problem before putting any money into repairs; you can go to a mechanic or shop that does diagnostic work, and have them inspect and test the engine to find the exact cause. But that may cost as much as $100; which is why I suggested just replacing the plugs first. Some probems are difficult to accurately diagnose; this is where the skill and experience of the mechanic becomes very important. That is why I suggest going to the most highly experienced and skilled person you can find; rather than shopping for the lowest price. A top flight mechanic may charge twice as much per hour as an inexperienced one; but the inexperienced mechanic may make repeated errors in judgment, and thus take more than twice as long to find the problem (if he even finds it at all). Some emissions inspection stations also do diagnostic and repair work; but there is a potential conflict of interest when a shop both repairs and certifies a problem. If the shop is thoroughly honest, it can be more efficient for them to do all the work; but if they are not so honest, they can easily run the charges up.
If you do not know of a mechanic you can trust, I suggest you check with the local Better Business Bureau, or ask at a NAPA auto parts store. You can also ask a policeman; as they often know about good mechanics.