In Graduation, Part 2 (s03e22) Faith says in her shared dream with Buffy “Little Miss Muffet counting down from 7-3-0” - the number being exactly two years from the end of Season 3 to the end of Season 5, when Buffy sacrifices herself for Dawn.
Dawn’s coming is foreshadowed again in This Year’s Girl (s04e15) in another Faith-Buffy dream as they prepare the bed for “little sis”. In Restless (s04e22), Buffy looks at the bed, saying “Faith and I just made that bed”. She then looks at the clock, the time reading 7-3-0 again. (Perhaps it should have been 3-6-5 with it being a year before the Season 5 finale, but there is no 3:65 on a clock).
The final references are made in Season 5 where it becomes clear that Dawn is ‘Miss Muffet’ and Glory is the spider.
It truly amazes me that Joss Whedon had already planned for Dawn’s arrival since the end of Season 3.