Whiplash nailed it:
The standard Lee 3-die set includes 3 dies:
1) Resizer / Deprimer - this squeezes the brass down to the correct diameter and knocks out the primer.
2) Case Mouth Bell / Powder drop - this flares the mouth open a bit to ease the seating of the bullet, and also allows you to drop in the powder charge.
3) Seating / Crimping die - this pushes the bullet into the brass to the right depth, and simultaneously, at the bottom of the stroke, crimps the case mouth tight around the bullet.
It is a bit tricking setting up a combo seating/crimping die, though not hard. Basically, you have to set the depth of the main body of the die in the die plate to give the correct amount of crimp, and you have to adjust the bullet stop so that it seats the bullet to the correct cartridge Overall Length desired. It's tricky because adjusting the crimp depth also changes the bullet seating depth, which then has to be re-adjusted.
By having separate seating and crimping dies, you can adjust these two steps independent of one another.