The space of solitude is completely different from a space shared by three people, the gains might be greater, but the difficulty increases exponentially.
Spiritual energy is everywhere, and even more, it's mixed together, creating a massive obstacle for the three to recover it. While receiving, they must first discern, then refine, and finally absorb.
To absorb someone else's foreign spiritual energy means you might be manipulated or influenced by them to some extent, and it might surface at an inopportune moment to cause trouble...
How can it be tolerated when the enemy resides in your Sea Of Consciousness?
The process is grueling, but all three are people of great resolve who will not easily give up, nor call a truce. There are many ways for cultivators to determine superiority, each process is different, yet equally deadly.
