The first three plagues punished the Egyptian gods, only from the fourth plague and on were the Egyptians themselves being punished. Therefore only from the fourth plague on was the difference between the Jews and the Egyptians very noticeable.

Vaeira Exile and Redemption (094) Likutei Sichos – Vol 01 p 119 ff The first two plagues represent parallel yet opposite ideas: Dam- Blood is to warm Egypt from it’s natural coldness to holiness, to be warmed by it. Tzfardea- Frogs were to cool off Egypt’s passions to worldliness. Here the order (is atypical:) first…