Tom: Sravana refers to hearing the teaching. Manana refers to reflecting upon and thinking that teaching which has been heard. Nididhyasana refers to prolonged meditation upon the Self, which culminates in Samadhi.
Here is a quote from Sri Ramana Maharshi taken from Talks with Sri Ramana Maharshi, Talk no. 249:
The effects of sravana may be immediate and the disciple realises the truth all at once. This can happen only for the well-advanced disciple.
[Tom: in other places Sri Ramana explains that this means that the advanced/ripe seeker immediately goes into Samadhi, which is the same as abiding as the Self, as soon as the teaching is heard]
Otherwise, the disciple feels that he is unable to realise the truth, even after repeatedly hearing it. What is it due to? Impurities in his mind: ignorance, doubt and wrong identity are the obstacles to be removed.
(a) To remove ignorance completely, he has to hear the truth repeatedly, until his knowledge of the subject-matter becomes perfect
(b) to remove doubts, he must reflect on what he has heard; ultimately his knowledge will be free from doubts of any kind;
(c) to remove the wrong identity of the Self with the non-self (such as the body, the senses, the mind or the intellect) his mind must become one-pointed. All these things accomplished, the obstacles are at an end and samadhi results, that is, Peace reigns.
Some say that one should never cease to engage in hearing, reflection and one-pointedness. These are not fulfilled by reading books, but only by continued practice to keep the mind withdrawn.
The aspirant may be kritopasaka or akritopasaka. The former is fit to realise the Self, even with the slightest stimulus: only some little doubt stands in his way, it is easily removed if he hears the truth once from the Master. Immediately he gains the samadhi state. It is presumed that he had already completed sravana, reflection, etc. in previous births, they are no more necessary for him.
For the other all these aids are necessary; for him doubts crop up even after repeated hearing; therefore he must not give up aids until he gains the samadhi state. Sravana removes the illusion of the Self being one with the body, etc. Reflection makes it clear that Knowledge is Self. One-pointedness reveals the Self as being Infinite and Blissful.