So basically, you can for example slot 4 fireballs on your level 3 spell list and that's all you'll ever need : for other spells, you'd rather use scroll versions. This way, you'll be able to throw 4 fireballs a day + a huge number of 3rd level spells through scrolls.
In 5e, you only slot Fireball once, and you can cast it as many times as you have third-level or higher slots. You can slot Fireball as well as other third level spells AND additional scrolls.
