This method comes in handy when you need to divide participants into smaller groups (in this case 3 people).
All the facilitators need to do is to tell participants this: “Find two partners and decide who of you will become Harry, who will become Ron, and who will become Hermione!”
That will create a sense of interest, and nobody will think too much about whom they want to partner up with. This way, participants will divide themselves much quicker and make everything easier for the facilitators!
Add other names from the “HP world” (or any other world) if you need groups of 4 or more people.