Commentaries:
The third plague, striking Egypt unannounced, hit the Egyptians in their religious ritual: They were a scrupulously clean people, believing that dirt and vermin made them impure and defiled. They bathed and shaved their bodies regularly to maintain their purity and superiority, but infected with lice, they could not worship in any temple. In addition, this plague struck the beasts also (verse 17), many of which were sacred animals kept in their sanctuaries. So, not only were the Egyptians defiled, but their gods and temples were defiled also.
Pharaoh's magicians could not duplicate this third plague in even a small way, so they admitted that Israel's God was greater than any of theirs (verse 19). God showed these pagan people that He controlled the creation and could do with it as He wished. The plagues on Egypt continued in this manner in sets of three, in which Moses and Aaron announced the first two and the third would descend unannounced.
Mike Ford
Animal Idolatry