The art of equalization is a subtle one. An experienced engineer can listen to a sound with such acute listening skills that he or she knows just what button to turn to get the sound perfect. Some of us aren't quite that skilled. Here's a great trick to find the frequencies that need help. (This trick works well on snare drums to find the frequency on the annoying ring that most snare drums exhibit.)
On a parametric or graphic (not a three-band) EQ, set the Q — the width of the EQ — as high as possible. This lets you pinpoint a very small range of frequencies, allowing a precise cut. Now set the level control all the way up as high as it will go, giving a maximum boost. Slowly change the frequency control, sweeping from the low to the high frequency. When you find the frequency that is responsible for the ring, or any other sound problem, you'll hear it easily. Because your EQ is boosted as high as it will go, that sound will jump right out of the speaker. Once you find the culprit, reduce the gain until the offending sound goes away. Repeat this as many times as you need to achieve the sound you desire. This is an absolutely essential trick as you learn about proper ways to EQ.
On a three-band EQ, you typically have no control over which frequency is boosted or attenuated. The three bands are set by the manufacturer and sweep through a general range of frequencies. This doesn't mean you can't use a three-band EQ effectively. If the frequency you need is built into one of the bands, than you're set.