If I recall correctly:
3 balls-20% of the Raid boss HP
2 balls-10% of the Raid boss HP
1 ball-5% of the Raid boss HP
It resets if you rejoin the battle after 6 mon are knocked out, but not if you stay in the lobby and not go back in (should really change tbh, leeching is really disgusting behaviour)
However, you can quit and go back in and still hit 20% for the 3 damage balls, provided that you have hit that 20% with the latest 6 mon that you have entered with.
Mathematically you can get the 3 ball damage even with a group of 12 (you only need to hit 20% of the damage and hope other people come in with Blisseys or other rubbish)
However, if a boss needs >5 people to defeat, then it's impossible to get a 3 ball bonus (Lugia definitely, maybe Articuno).