The 3-die roll crimp set is for loading straight-wall cartridges. Its steel sizer die sizes the case and de-primes. Note - cases must be lubricated. The expander die expands the case to the proper diameter to accept the bullet, and it imparts case mouth flare/bell for bullet feeding. The seater die seats the bullet and roll crimps.
Group C - straight walled rifle cartridges
3-Die set with roll crimp