A redated post.
Here's what I wrote about the LLL argument in my book. pp. 13-14.
Perhaps Lewis's most famous argument is what is known as the Lewis trilemma. It is unreasonable, Lewis says, to say that Christ is a great moral teacher but not God, because he claimed, both implicitly and explicitly, to be God. If he wasn't God, he either had to be lying, which would make him wicked, or he had to be deluded, which would make him insane. Since these two alternatives are implausible, Lewis says he must be telling the truth and really be God.
Many others have repeated this argument in their own apologetics. The argument makes four assumptions, however, and critics of the argument have challenged all these assumptions.
1. Jesus' claims in Scripture are best interpreted as not merely as claims to be the Jewish Messiah, but as claims to be God.
2. The Gospels are a reliable historical record of what Jesus said and did.
3. No sane person can form the false belief that he himself is God.
4. The claim "Jesus is God" is more antecedently probable than the admittedly antecedently improbable claiim that Jewus was a great moral teacher but and either a liar or a madman.
Lewis supplies some argumentation in defense of all of these claims in various parts of his writings, and it seems to me that there is a good deal to be said on both sides of each of these claiims before a final assessment can be reached. If all these assumptions are defensible, then the argument is a good one. But rather than debating these assumptions, apologists have simply repeated the mantra "liar, lunatic or Lord," while opponents have cried in response "false dilemma." Neither of these responses, in my estimation, does justice to the complex issues the trilemma raises.