Shafer-Landau’s three objections to moral realism, philosophy homework help

Explain Shafer-Landau’s three objections to moral realism? Speifically, the intractability of ethical disagreement sustains an antirealist diagnosis of ethical thought and talk. And I am referring to his work ” Ethics as Philosophy : A Defense of Ethical Nonnaturalism”

 (please explain in the simplest terms so it’s easily understandable)