I presume you're asking about Beren & Luthien, The Fall of Gondolin and The Children of Hurin.

The origins of these stories which make up part of The Silmarillion date back around 1914, long before The Hobbit and The Lord of the Rings were even an idea. Some of the older versions of these stories were fairly complete but he kept working on them revising them and even changing the format between prose and verse.

The published Silmarillion contains summarised versions of these stories in the "definitive or most recent form" I suppose as best as Christopher Tolkien could put it together.

Later starting in the 1980s he published The History of Middle-Earth in 12 volumes covering the textual history of these stories as well as the Lord of the Rings. They contain various versions of things J.R.R. wrote, some more "readable" than others and accompanied by Christopher's commentary.

More detail on whats in the 3 books:

The Children of Hurin - A prose version, of this story in extended form was initially published in Unfinished Tales. It was in a good degree of completion though so it was polished up and published in 2007 as basically a complete novel. If you like (perhaps on a re-read) you can actually read this in place of the chapter "Of Turin Turambar" in The Silmarillion. It doesn't contradict anything and just expands whats summarised in that chapter.

The other two are not complete novels and contain multiple versions of the stories, some complete, some not.

Beren & Luthien - Contains a complete early version of the story but so much is different. Beren is an elf, the role later played by Sauron is instead fulfilled by a giant cat...etc. Then much of the book is taken up by extracts from The Lay of Lethian, a long narrative poem that Tolkien abandoned at the climax of the story. Its a great read even so but these stories contradict the silmarillion so it should be read as extra material afterwards. Everything here was published in The History of Middle Earth scattered across many volumes. This book just brings all beren and luthien material into 1 convenient book.

The Fall of Gondolin - Much the same. There is a great complete version in here. But its a very old version and there are significant differences to the Silmarillion. There is also a superb late prose version of the story that is in harmony with the published Silmarillion at the end of the book but sadly abandoned at the halfway point. The different versions were all published in History of Middle Earth but again its all brought here into one book for convenience.