All three words or phrases described here, are taken from/recorded within The Yoga Sutras of Patanjali.
Dharnaa, is a sankskript word meaning “focus” or “concentration”. It has different translations and different yogis tend to teach the concept slightly differently.
Some say it relates to the focus on what you want to attain, and what you want to be. Dharnaa practice, involves concentrating on the specific subject or matter you want to focus on and become. For example, a guru might give a student a specific mantra to use, that has a specific meaning.
You can also practice Dharnaa by focusing on a single object, like a candle’s flame. You can also focus on the breath, or eyebrow centre.
Dharnaa; the ability to focus, is thought to be a fundamental skill, so you can go on to attain higher levels of focus and personal development along the yogic path (well, most paths). It’s hard to meditate for example, if you are unable to focus. Dharnaa should be the first goal, or practice before any other form of meditation.
Dhyāna means contemplation, or sometimes it is used to describe the practice of meditation. It is regarded as an important element of the “self-knowledge process”.
Vivekananda explains Dhyana in Patanjali’s Yoga Sutras:
“When the mind has been trained to remain fixed on a certain internal or external location, there comes to it the power of flowing in an unbroken current, as it were, towards that point. This state is called Dhyana”
Dhyana is also said to be the seventh limb of Astanga Yoga. It is a continuation, or next level up from Dharnaa.
Samadhi, is ultimately the goal of meditation and yoga. Although, “desire is a trap”, so you can’t really yearn or aim for this goal as it will paradoxically impede your development.
In the Yoga Sutras, Patanjali describes “samprajñata samadhi” / Samadhi with higher knowledge and “asamprajñata samadhi” / “beyond higher knowledge”. There’s a great article on YogaInternational.com here.
Jnaneshvara Bharati describes asamprajñata samadhi below:
“The other kind of samadhi is asamprajñata samadhi, and has no object in which attention is absorbed, where only latent impressions [samskaras] remain; attainment of this state is preceded by the constant practice of allowing all of the gross and subtle fluctuations of mind [vrittis] to recede back back into the field from which they arose.”
Quote taken from ocoy.org