Easy Peasy

The guard ordered the numbers from small to big, then escorted the three of them up to the stone door at the cliff. They entered to find that there was nothing in there whatsoever but stone walls. 

"This is balance training. In a moment, I'll leave and shut the stone doors behind me. Then, I'll press the secret switch to start the training. Three hours later, it will shut down," answered a strong guard in a respectful tone of voice.

"Yes, I understand," replied Liu Rushuang lifelessly. 

The guard glanced at the three people who looked ready to train, then walked out.

After the stone doors shut, the floor suddenly withdrew from both sides. A stone column appeared in the center and Liu Rushuang jumped onto it, calm and composed.

Number Three and Number Eighteen copied Liu Rushuang and stood on top of the two other stone columns that revealed itself from the ground.