Bearing in mind that all the Egyptians' animals died in Egypt (See Va'eira 9:6) and Yisrael took all their animals with them (See Bo 10:26), whose animals did they use to harness their chariots?
Rashi and Targum Yonasan: They took the animals of the G-d-fearing Egyptians who took their animals inside (See Va'eira 9:20). 1
R. Shimon had this episode in mind, when he stated 'If you come across the most righteous Egyptians, kill him, the best snake, crush its skull!' (Rashi).
What is the meaning of "Shalishim"?
Rashi, Rashbam and Targum Onkelos: It means that each chariot was under the command of an officer (a) seasoned warrior. 1
Targum Yonasan: It refers to Pharaoh's innovation of adding a third horse to each chariot, to increase its speed.
To whom the Torah refers later (See 15:4 [Rashbam]). See also Seforno.