EKG Help: 3 vs 5 vs 12 lead

3-lead is usually used on transport monitors, and monitors two different areas of the heart (one lateral, two inferior).

5-lead is preferred in an ICU, to monitor the third (anterior) area. For example, if your pt is admitted as a r/o MI (rule out Myocardial Infaction = suspicion pt is having a heart attack), with the 5-lead you can keep an eye on the three areas of the heart and if you see changes in any of the leads (especially if accompanied by chest discomfort or vomiting or VS changes), you can (per protocol, hopefully) get a 12-lead ECG.

12-lead ECG gives a more detailed look at the heart's three areas (anterior=front, lateral=side, inferior=back), and changes in certain segments of the ECG in the related leads for each area suggest the area of concern. For example, ST changes in leads II, III and aVF may suggest a problem in the right coronary artery.

Hope that helps. I'm sure others may chime in; getting different ways of explaining things is helpful.

We also use 5 lead EKG in PCU. Lead II is typically used to determine HR and to identfy a P wave, and the chest lead can be placed over a specific area of concern, say heart muscle supplied by a newly stented artery.
12 lead EKG is also used to identify Axis and specific bundle branch blocks. You may have an irregular rhythm with a wide QRS complex--a 12 lead can be useful in distinguishing a fib with aberrant conduction from v tach.