Surrender And Accept Civilization.

The resistance force expected an army so they were waiting for more soldiers to come through the portal. But no one came after the first three. 

"Maybe they sent negotiators," They thought to themselves.

That is a valid explanation for why just three enemies have entered the plane instead of an army. So they focused on the three enemies. Not only are they few in numbers, but they are also underwhelming to look at. They look like white dolls without hair, eyes, nose, mouth of any kind, lips, or ears. They are humanoid with two arms and two legs but they are featureless. They are so smooth they are creepy. 

The three of them stood together in the air with their arms crossed as they overlooked the resistance of 2 billion arrayed before them. The robes they wore fluttered in the wind. There was no concern on their face. Then again, they have nothing on their face so there's nothing to indicate their emotions.