In 4:9 we find Hashem had just performed two signs for Moshe for him to relate to the people to gain their trust. Now the passuk says, and if they still do not believe you here is a third trick, as He explains to Moshe to turn some water from the Nile to blood in front of them.
if they do not believe after the first two, why would they believe after the third?
And anyways, why would any "magic trick" help to gain their trust - they are living in Egypt. Egypt is the center of the world for magic and black arts. Why should they be impressed just because Moshe can pull off a couple of stunts?
I would say it was not the magic that was important to sway the people. Anybody could do magic. It was the symbols and meanings behind it that were important. The third trick is one where Moshe takes water from the Nile and turns it to blood.
Moshe, by doing that, is openly and brazenly attacking the national symbol of Egypt. Rashi goes further and says Hashem was sending a message by this trick that He would attack and destroy the gods of Egypt first (the Nile was respected as a god).
The idea si that the specific action had meaning and was not just a simple magic trick. Each of the three tricks had increasingly powerful messages, so someone who had more faith might be swayed at the first trick, someone else at the second, and those who still did not believe after two tricks, would definitely be swayed by the third, with the most powerful message. No matter how stubborn they were, they could not ignore message #3.
Moshe being brazen enough to attack Egypt at its national symbol is a powerful message.