The three principal causes are the things that shore up your practice. They all provide support for your practice, and it is important to recognize them. In addition, it is important to realize that many people do not have this kind of support in their lives, and to consider ways to provide such support for people who need it.
The first principal cause is that of the teacher. Whatever your interest, custom, or practice, it is something you learned from a long line of teachers that extends back to the earliest days of mankind. From the earliest teachers to the internet, your ability to practice dharma, or yoga, or any other discipline is due to the hard work of generations of teachers. Consider that in certain areas you may be a teacher or potential teacher to others.
The second principal cause is to remember the significance of our ability to train our minds and our bodies. The fact that there are systems of mind training developed in ancient times that are still in use points not only to their efficacy but toward our own natural ability to train our minds. Meditation, yogasana, lojong qi gong: all of these as well as other systems exist to help us train our minds. It is up to us to make use of them, and to practice them with discipline, which, after all, is the difficult part.
The third principal cause is to have sufficient support to undertake and continue one’s practice. What this means is that you need to have your basic needs covered: shelter, food, clothes, and some kind of community, no matter how small. This is an important one, because many of us (me!) can fall into a mentality that, as spiritual seekers, we should shun money and material possessions. Indeed we should shun them for their own sake, but we have to recognize their ability to bring the circumstances into our lives that support our spiritual or creative practices. The trick is in not allowing ourselves to be fooled into thinking that possessions are the reality of our lives. In any case, Buddha never advocated living in poverty, nor in opulence, espousing the middle way in all things.