Traditionally, their lies are usually woven around a kernel of truth. That is, Anduin might not realize he is telling a falsehood, but he will give us 3 of them. Note that Ilganoth did not even say that Anduin would lie to us, which would imply that Anduin had the intention of misleading; instead he said "Three lies will he offer you" which sounds more like he could be passing on bad information that he believes himself.
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Yep. Just because he will offer us three lies, that doesn't mean that he will knowingly mislead us even once. Only that he will be duped into sharing bad information. One, for example, could be that Sylvanas was responsible for Teldrassil. Anduin could be deceived into believing it was true, when really she was framed.