Every time you click on a fic on ao3 while logged in, after a certain amount of time it will show up in your history.
You can find it by clicking on ‘My History’ while hovering over your username.
On this page you will also find some other options.
You can mark a fic for later by selecting the button from the top of a fic page. Pressing it again when it says Mark as Read removes it from this list.
On the homepage, when you scroll down, ao3 will list 3 random fics from your marked for later list under this heading:
