רמב"ן — מה זה בידך / הבא נא ידך בחיקך

Q — Why were the first two signs (rod-to-snake, hand-to-tzaraat) performed for Moshe at the burning bush, when the third (water-to-blood) was reserved for performance before the people? Moshe didn't need signs to believe Hashem was speaking to him.

A — Ramban (following Shemot Rabba): the first two signs at the sneh were not for belief — they were a hint and a punishment for Moshe's lashon hara about Israel ('והן לא יאמינו לי'). The nachash echoes the original lashon hara of the primordial serpent; the tzaraat is the standard punishment for lashon hara. Moshe fled the snake ('וינס משה מפניו') fearing the punishment, even though he knew the outcome was in Hashem's hand — 'כל אדם מתרחק מן המזיק לו'. The third sign needed no such rebuke and was performed before the people for their belief. (Alternative: the signs strengthened Moshe's heart that Hashem changes nature, and only two were needed; water wasn't available at the sneh anyway.)