Meruem's aura far succeeds each of the Royal Guards, and even if they fought against him with good teamwork, their power could not exceed him. Maybe they would give him a good fight considering they have strong Nen abilities. But even their power would not suffice to kill him. Moreover, Meruem is extremely intelligent, and he understands his Royal Guards and their abilities very well. Hence, he could quickly develop a strategy to take all three down during a battle.