Suppose a third firm enters the market. If they all don't cheat, then they split the profits three ways so that each firm gets 2 million. If one firm cheats, he steals profits from the other two firms and earns 5.5 million while the other firms incur losses of 1 million. Will it be harder or easier to sustain cooperation?

Harder to sustain cooperation

Explanation:

When the number of firms in an oligopolistic market increases as is the case here, cooperation becomes harder because profits become smaller so companies start to become more selfish.

Also in this scenario, the cheating firm has the incentive to do so because the profit is so much higher than sharing so companies will always be looking to cheat in order to make this profit. The other firms would reply by reducing prices and all of them would suffer.