I doubt the bold. He was using what his father left
Not exactly:

http://www.worldcrunch.com/culture-s...lly-speaks-out

First in England, then in France, he reassembled the parts of The Silmarillion, making it more coherent, added padding here and there, and published the book in 1977, with some remorse. "Right away I thought that the book was good, but a little false, in the sense that I had had to invent some passages," he explains. At the time, he even had a worrying dream. "I was in my father's office at Oxford. He came in and started looking for something with great anxiety. Then I realized in horror that it was The Silmarillion, and I was terrified at the thought that he would discover what I had done."
If The Silmarillion, it is not a trilogy either.
The Silmarillion is actually a quintuple tale: the Ainulindalë and Valaquenta; the Quenta Silmarillion; the Akallabêth; and The Rings of Power and the Third Age. The first two (the story of the creation of the Ainur and accounts of the Valar) would serve as an intro to--- the Quenta Silmarillion (part I of the trilogy...the story of the Silmarils); the Akallabêth (part II of the trilogy...the rise and fall of Númenor); and finally, part III, the Making of the Rings of Power and the forging of the One Ring.

A trilogy, yes?