What is the correct order of processes in the accretion of planetesimals?

Masses of matter form around dust grain, masses of matter collide, and atoms and molecules are added to dust grain.
Masses of matter collide, masses of matter form around dust grain, and atoms and molecules are added to dust grain.
Atoms and molecules are added to dust grain, masses of matter form around dust grain, and masses of matter collide.
Masses of matter collide, atoms and molecules are added to dust grain, and masses of matter form around dust grain.