In 1e, 2e, and 3e when the spell progression table said you got 3 first level spells, then at the start of the day you decided which spell was placed in each slot. If you picked, Magic Missile, Magic Missile, and Detect Magic, then the three first level spells you could cast for that day would be Magic Missile, Magic Missile, and Detect Magic exactly. You couldn't choose to cast Magic Missile 3 times, or Detect Magic twice.
The exception was Sorcerer in 3e. That class choose what spell to use for each slot at the time of casting. This was called "spontaneous casting" because Clerics have an ability that basically does the same thing, but only for cure spells.