The number three appears all over in Western history and thought. The trinity is a fundamental concept to Christian theology (father, son, holy ghost), our view of time is broken into three parts (past, present, future), philosophers often break their thoughts down into "trichotemy" (Plato, Aquinas, Kant), a musical chord is made up of at least three notes, psychology speaks of three part psyche (id, ego, and superego), and all throughout other fields (art, science, geography, etc), the number 3 holds great significance.
However, perhaps the greatest significance it has, and the reason why this simple number has become so important to us, is that a traditional viewpoint of the self consists of three parts: the body, mind, and soul.
Despite my lack of firm spiritual beliefs, and perhaps despite some logical beliefs to the contrary, I still hold to the concept of the three part self. I do, however, prefer to eliminate the soul/spirit from the equation because of supernatural undertones these two words provide; instead, I will refer to this third part as the psyche. This not only does away with some of the supernatural views baked into the very world "soul," but it's also what the general scientific community refers to this part of us as (especially in psychology), thusly it seems more accurate in multiple ways.
To quickly summate this view: the body is our mass in the world and consists of physical needs/desires; food, exercise, sleep, sex, and the like. The mind is the logical, analytical, mechanical center of our being; for it, we need education, instruction, and challenges to keep us growing and expanding our knowledge of existence. And the psyche is the subjective, emotional, individual, creative outpouring; compassion, benevolence, love, and community are aspects of what keeps this part healthy.
Each part of the self needs to be stimulated and cared for in their separate ways if we are to be a balanced person. By giving preference to one over the others, we will become weak in the others and sink the whole ship. Just as one bad apple can ruin the whole bushel, one ignored or preferenced part of the self can, and will, ruin the others.
We have all seen someone like this in our own lives, I'm sure. People who over-indulge the bodily desires, in both healthy and unhealthy ways, are typically not pillars of knowledge. The stereotype of a jock comes to mind here as someone who is so focused on their body that they forget to feed their mind, and often their psyche as well, and become lopsided beings. People in this extreme are often shown in our media as being trapped in their past, eventually letting themselves go and is never a whole or healthy person.
Those who over-indulge the mind are often of impressive prowess in their fields, but do not hold their own physically and are often awkward in communal situations. This geek/nerd stereotype is often seen as not having downsides (because they go on to make tons of money and get the girls through their wallets), but having known many of them myself, I can definitely say that their psyche and emotional health suffers greatly. They are less open with others, hesitant to trust, have emotional scars from bullying that was never dealt with, and have a hard time relating to other individuals because they are so caught up in their own world that it becomes too hard to imagine another persons situation.
And those who are too focused on the psyche, often are neither logically sharp or a bodily powerhouse. Perhaps this stereotype is best fit to that of the thespian, the poet/writer, or the musician. They are in commune with others regularly, they love much more feely, and wear their emotions on their sleeves... however the realms of critical thinking and logic are often completely foreign to them. These are the free-spirits, the hippies, those who take the path of least resistance. They may very well be smart and fit, but not healthy in either area.
These are all exaggerated examples and stereotypes, but looking over our lives and across our friends, I'm sure we can see inklings of these blown-up personalities in ourselves and those around us. Most people do not throw themselves completely into one area of life, although it does happen from time to time, but we can see how even placing a good deal more into one area can lead to suffering in the others.
Life is a balancing act between these three selves; there is no perfect equation to be given for how to live one's life because each of us are different, and, each of our equations is constantly in flux. Every event, decision, relationship, and circumstance of life, whether our own or just those around us, impacts our world on a monumental scale. Therefore, we must not get too comfortable with any one solution, as it will only be the solution for a tiny, finite amount of time.
With our world and our lives constantly changing, this means that we should be changing as well; if we do not keep up ourselves and our life-experience in tandem, we will surely become an unstable, lopsided person who wobbles about instead of standing firm and secure.
We must know ourselves and what we need in each area of the self before we can begin to understand or help anyone or anything outside of us. Until that point, our internal confusion and lack of balance will only obscure our vision and taint our perceptions, giving us an untrue judgment of the world.