This is probably the hardest of the three to describe. What does it mean to be simple and what is simplicity? Simplicity may be seen in context to some opposites: Duplicity & Complexity.
Duplicity points toward cognitive dissonance - a disagreement between our thoughts and actions, or our mind and body, or our words and our actions, or even between two disagreeing ideas within our own mind. To be simple is to get everything in agreement. Live as you know you should. Make your actions follow your thoughts. Say what you mean. Be who YOU are. Be TRUE to yourself.
Complexity is the tendency to live far away from what you need to live. As Gandhi was famous for saying, "Live simply that others may simply live." This is FAR from the case now in nearly everyone's lives. Ask yourself a few questions -
What do I really NEED to live?
Do I have far more than I need?
Are the other things I think I need (beyond basic food and shelter) really needed or just a distraction?
How much love, community, communion do I have in my life? (Now days most people have very few close friends to whom they can divulge everything and with whom they have a deep heartfelt connection - one of the few things besides food and shelter that IS necessary for most people.)
How many people/environments/species is my existing/consuming negatively affecting? (Probably far more than you realize.) You see, nearly everything you do/have/buy/eat has an impact on the globe, and far more often it is negative rather than positive.
In what ways are you trying to be less of a burden (yes BURDEN) to the planet and ALL its inhabitants?
How much of your time is free to contemplate 'higher' things in life - love, beauty, truth, the Absolute - as opposed to thinking about how you will pay off the innumerable bills for things you probaby didn't need in the first place? (The two happiest periods of this life - Now, as I simply sit on my porch at Barnashram, and when I lived as a monk with 9 other men in a 10x12 foot room.)
ARE YOU HAPPY? ARE YOU PEACEFUL? ARE YOU SATISFIED?
Do you really need the things that will make your life bigger, better, faster, more, and so on, or are these things really just additional burdens? You see, simplicity is a form of freedom. It is a lifestyle of freedom. And it not only helps you to be free, but helps to release others in the process. Stop seeking and start being, or as we like to say:
"STOP AND BE PRESENCE"
Or as my teacher used to talk about "Simple living - high thinking." The less you are engaged in the seeking of material gratifications, the more time there is to simply be and be happy, to inquire into Truth, be and share Who you are.