Why did Yehoshua need to begin with Terach and his homeland?
Malbim: From Adam until Avraham, the Shechinah rested only on individuals. They were the heart and fruit; other people were shells. Starting with Avraham, Hashem wanted to put Shechinah on all the Shevatim and an entire nation, to whom He spoke [on Har Sinai]. This required many refinements, tests and casting away shells, for Avraham's family did not live in a Kadosh place, and they were idolaters. This is why it mentions (verse 3) bringing Avraham to Eretz Kena'an.