How can I randomize players between 3 teams, while there being no chance of no players on 1 team?

So, I want to randomize players between 3 teams based on a percentage. Green Team: 70% chance Red Team: 20% chance Blue Team: 10% chance

I currently have a math.random setup, randomizing players based on those percentages, but that means that there is always a chance that no players will spawn on 1 team. How can I set it up so that at least 1 player always spawns on each team?

You don't have to provide code if you don't want to, but only explain your theory. Thanks!