Why does the Torah use three different expressions for the hardening of Pharoh’s heart in Parshas Vaeira?

The hardening of Pharaoh’s heart is referred to no less than twenty times in the course of the story of the Exodus. Sometimes it is Pharoh who is said to harden his heart. At other times, Hashem is said to have done so. The Torah uses three different verbs in this context: chazak, to strengthen, kasheh, to harden, and kobed, to make heavy. Horav Shlomo Miller’s Shlit’a opinion is that it depends in the case and context involved