In the third quatrain to what does the poet refer in "Sonnet 18"?
But thy eternal summer shall not fade
Nor lose possession of that fair thou owest;
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou growest:
In the speaker's comparison of his love to a "summer's day" he speaks mainly of her youth and beauty. The first two quatrains go on about the different parts of the actual summer that could be considered ugly or imperfect: the wind shakes the buds, summer as a season is far too short, at some point everything that is beautiful will eventually lose its beauty, etc.
By the time he gets to the 3rd quatrain, he refers to his love's timeless youth and beauty. The first two lines speak directly of her eternal youth and beauty that she will never lose. The third line says that she will not die. The fourth line explains this immortality, which is not literal. The speaker will keep her alive in his poetry.