Ex 4:1 Then Moses answered and said, "But suppose they will not believe me or listen to my voice; suppose they say, 'The LORD has not appeared to you.'"

 2 So the LORD said to him, "What is that in your hand?" He said, "A rod."

 3 And He said, "Cast it on the ground." So he cast it on the ground, and it became a serpent; and Moses fled from it.

 4 Then the LORD said to Moses, "Reach out your hand and take it by the tail" (and he reached out his hand and caught it, and it became a rod in his hand),

 5 "that they may believe that the LORD God of their fathers, the God of Abraham, the God of Isaac, and the God of Jacob, has appeared to you."

 6 Furthermore the LORD said to him, "Now put your hand in your bosom." And he put his hand in his bosom, and when he took it out, behold, his hand was leprous, like snow.

 7 And He said, "Put your hand in your bosom again." So he put his hand in his bosom again, and drew it out of his bosom, and behold, it was restored like his other flesh.

 8 "Then it will be, if they do not believe you, nor heed the message of the first sign, that they may believe the message of the latter sign.

 9 "And it shall be, if they do not believe even these two signs, or listen to your voice, that you shall take water from the river and pour it on the dry land. And the water which you take from the river will become blood on the dry land."

Moses was concerned about his credentials. And so, in order to establish it before the eyes of the Israelites, God granted him three signs. Three, in order to establish and demonstrate it without a doubt.

The first sign was of a serpent. His rod turned into a serpent and yet it turned back into his rod again when he took it by the tail.

Serpents represented the power of Egypt. The Pharaohs of Egypt would wear a metal cobra around their heads symbolizing the nation and its power. Moreover, it was worshipped in Egypt.

The ability of Moses to turn the serpent into his useful rod would encourage confidence among the Israelites in Moses. It would truly demonstrate that God was with him. It would send a message to the Israelites that this Moses can handle the power of Egypt.

The second sign was that of leprosy suddenly infecting his hand and yet was immediately healed back to health again.

Probably, this was Hansen's disease and it was incurable during their time. The gods of the Egyptians were helpless against leprosy, and, yet, here is Moses, with the ability to deal with the disease just like that. It would demonstrate that God was with him and that his God was powerful over the gods of Egypt.

The third sign was that of water from the Nile turning into blood.

The Nile was the source of life for Egypt. The entire nation was dependent on it for its survival. The Egyptians attributed the benefits of the Nile with their god Osiris.

Moses' ability to turn the water of the Nile into blood was a sign that the life source of Egypt was in his hands. He got Egypt by the throat, so to speak. And this would demonstrate the power of God  over their god Osiris and that this God was with Moses.