What were the main Upanishads and Vedanta subjects that Maharaj gave discourses on during his three daily sittings?

Contrary to what the question assumes, the provided information states that Maharaj did not give such discourses. As mentioned in Chapter 0, 'Maharaj did not openly explain profound Vedanta subjects or give discourses on Upanishads.' His approach was different; his advice was primarily ethical in nature. During the three daily sittings, he would give advice through stories, which served to address the various matters on the minds of the devotees present. The purpose of this method was to first purify the disciple's ground before sowing the seed of higher knowledge, which is considered the proper practice for a Sadguru.