Actually are 3 at the same time. Im very bad at making combos. I did cleared it 1 time and was able to pass to the final stage but I use Lilith on the 4 round. The I died on the 5 round so I used a stone to be able to pass it.
But now im farming for the mystic mask and i know is not a 100% drop.
Combo is not important, the important thing for killing 3 at the same time is groups of 5 with your strongest color, even if there is no combo at all. This is also reason why you want as few colors as possible. For example, if every monster on your team is dark, and you match 5 black orbs in 1 group, you will hit every monster on the screen for 6 HP.
If you have multiple colors of people in your group, then you have to try to make 1 group of 5 for each color that is on your team. This is why you store some orbs in the corners in the beginning, then try not to match those stones until you need them. Make a shape that looks like this:
Where O is the color that you have the most people from in your group (if you're all one color, even better). From here, if you are dragging an O stone, you can make a group of 5 O's on the bottom line easily, and on the next turn you can make ANOTHER group of 5 O's on the bottom.
If at any point you don't have enough stones to make a group of 5, you use an orb change of the appropriate color. Lilith for example can use orb change. None of the other guys you brought with you can, but replace one of your pengdras with a low level orb changer. If you can change your red pengdras to blue pengdras, and your red ogre to blue ogre that would be better too, because siren is blue so you want as much blue as possible if you can. This would give you 2 colors instead of 3, so easier to do large damage.
so for example, maybe you change your team to this:
blue ogre, 3 blue pengdra, lilith, siren.
Now if you match 5 blue stones you will do 5 damage to every monster on the screen. An even better group is if you can replace 1 or more of your blue pengdras with blue monster who can also change orbs to blue. Mystic Ice Knight / Fenrir Knight is a great example. Not sure if there are others