Why, only for this sign, did Moshe first have to conceal his hand?
Maharal (Gevuros Hashem Ch. 27, p. 111): The first and third sign involved an action on Moshe's part; whereas this second sign did not. To initiate the miracle, he was to hide his hand, to bring about the change from a concealed place.
When they later came before Pharaoh, Moshe (and Aharon) would perform only the first sign, of casting the staff (which turned into a Tanin - 7:8-13). Why not the other two signs?
Maharal #1 (Gevuros Hashem beg. Ch. 32, p. 121): Before Am Yisrael, Moshe was to perform all three signs, to help them reach full Emunah. Pharaoh, on the other hand, was not going to believe in any event! It was sufficient to perform one sign (for he had requested it).
Maharal #2 (ibid. Ch. 27, p. 110): The three signs represent three stages in Hashem's justice against man; refer to 4:3:3.1:2. The first sign came to instill fear - and fear would be sufficient for Pharaoh at that point in time (for if the Egyptians were frightened by the Tanin, they ought to fear Hashem, who had created it!). Ultimately, the Egyptians would be taught the other two lessons the hard way -- they were stricken by the Makos, and dealt a death-blow at the Yam Suf.
