Dear young madam,
There are no three types of hearth rythm, but there are many. Maybe the idea of three refers to Sinus rythm, atrial and AV node rythms, and ventricular onset rythms. Possibly 'three' refers to vagal rythm, normal and adrenergic; I don't know. So, it would help if you indicate where the idea of three comes from to see what answer you are looking for.
Your second question is easy to answer: the best way is an EKG: an elecrocardiography, often called a 'heartfilm'.

The following videofilm is a nice introduction.: http://video.about.com/heartdisease/Conduction-System.htm
After this you grasp that Sinus rhytm is normal, 'Atrial' rythm originates one step lower in the system and ventricular rythm originates even further down, in the wall of the main chambers of the heart. But again, I don't know if this is the differentiation you are looking for.

Maybe you are even looking for a very different classification system. So called 'hearth math' refers to beat-to-beat regularity. A small device calculates the distance between every beat. Supposedly, the heart becomes better harmonised, depending for example on psychological stress. We recently exercised psychodrama psychotherapy and participants had a small device ateached to their ear. We created a scene where one participant played my 'heart' in it's psychological meaning. So, heart as it is used in the Bible, a place for deep emotions and the like. It was interesting to watch that our collegue, when se took a seat in the chair that was my heart in this psychological sense, depicted optimal heart variability when connected or enliving this 'heart' quality. In a sense, there are as many hearth rythms, as there are human beings.

Sincerely, Johannes Schilder, Emergency Physician and psychodramatherapist NVGP