What was the third plague?

Answer: Gnats (lice)

Aaron struck the dust with his staff and it became gnats (KJV: lice) on man and beast throughout Egypt.

“Then the LORD said to Moses, “Tell Aaron, ‘Stretch out your staff and strike the dust of the earth, that it may turn into swarms of gnats throughout the land of Egypt.’””— Exodus 8:16 (BSB)