The Celestials can kill all three of them but they will lose in this exchange as long as one of the Celestials dies. Gods grow stronger by absorbing compatible domains but demon kings grow stronger by absorbing Celestial Godhoods. That means demon kings can absorb any Godhood as long as it belongs to a Celestial.
Every Celestial is food for a demon king. They are not picky at all. It makes them grow stronger faster than Celestials. What makes things further in their favor is that dying only makes them lose some of their Authority, not their strength.
Each new demon king is a level 1 demon king. They need to absorb 3 Godhoods to become level 2. Then 5 more Godhoods to become level 3. Then 7 more to become level 4. Then 9 more to become level 5. Then 12 more to become level 6. That means each demon king has absorbed at least 24 Godhoods of Celestials to become level 5 demon kings. They might have more than that.
Death means the loss of a single Godhood. A new level 5 demon king will be relegated back to level 4 if they are killed. Their death will reduce their amount of Godhood from 24 to 23 thereby causing their relegation. But these demon kings have more than 24 Godhoods of Celestials. They might die but their strength will be preserved at level 5.
The Celestials have decided to work together to grind down the demon kings. They will win when they eventually kill the demon kings to a level where they are too weak to be a threat. But they will lose as long as a single one of them dies. That will reduce their overall strength and make it easier for another one of them to be killed again when the demon kings return for the second round.
The challenge for the gods is to keep their strength constant throughout the duration of the war. This war can continue for many origin cycles. So the demoness of lust was right. Their conflict is a race. Hitting hard and fast is good sometimes but this race is one that can only be won by being slow and steady.
The gods have to work together or they will be divided and subsequently conquered. That's why they came together as one. The three demon kings on the other hand did not fight as one. They have less to lose so they are not as worried. The major reason why they are not fighting together is because it is very difficult for them to be team players.
The snake demon licked its lips before shouting, "Come and get a piece of me."
Then it shot forward. It flapped its wings of red fire and soared into the air against the 10 gods. He can't wait to fight already. Aeternus was the one that suggested that they wait. The demon king of lust agreed with him so it was two against one. Unfortunately for them, that isn't enough for the demon king of wrath to hold back. If it were up to it, then they would have attacked earlier instead of being attacked. The sin of wrath is not one with patience and it likes to be proactive in a battle instead of being reactive.
The succubus queen shook her head in mirth. "It seems he has forgotten the battle plan. He is so spontaneous and exciting. How about you? Are you also exciting?" She leered at him.
"We stick to the battle plane. Follow my lead." He told her.
He wasn't troubled too much that someone already disregarded his carefully laid out plan at the beginning of the battle. He reminded them several times to stick with the plan but this still happened. He wasn't concerned though. This is just a slight hiccup. It is expected when working with demons. He can't expect them to be as united as the gods. It is already a success that he was able to avoid infighting among them before the battle.
Aeternus flapped his four black wings and shot forward. His speed was so fast that he overtook the snake demon quickly. He blazed forward and left a dark smoke trail behind him. The Celestials stayed in tight formation as the two sides approached each other. They did not let the fact that the demon kings are not acting as one let them relax their guard. They will only relax when all three of them have been eliminated.
The snake demon complained as Aeternus passed it by. It yelled, "Not fair."
Then it seemed to have realized something. Things were supposed to go this way so it shouldn't be complaining. It shut its mouth and put in more effort in keeping up with Aeternus. It flew faster and stuck to the smoke trails that Aeternus left behind. Aeternus left his partners way behind. He advanced forward like an unstoppable comet. He didn't seem like he intended to stop. His bare face was expressionless as he continued on his flight path without slowing down.
The Celestials on the other hand began to frown. They will collide with him if they continue on their current path. They have to either divert their path and make way for him or they will have to block him. They don't want to make way for him because they are a group. There are 10 of them and one of him. The one that should make way is him, not them. So they decided to block him. Even better, they decided to attack him. The best defense is an attack after all.
The Celestial Supreme said to the other Celestials, "We will continue forward no matter what. I will slow him down and we will gang up on him. It is best that we kill him before the other two catch up. Let's do this quickly and we will have a semblance of peace in a few hours."
They voiced their assent and continued to follow their leader into battle just as they have always done. He has brought them victory before and he will surely do so again.
---------
A/N: Bonus chapter for 600 votes.