Who is "Tzemach"?
Rashi #1: It is Zerubavel, like it says above "Hineni Mevi Es Avdi Tzemach" (3:8). He is called so because his grandeur was Tzome'ach (sprouted) little by little 1 . Radak - in the end, his level will be very great - "Ekachacha Zerubavel..." (Chagai 2:23). He will be with you. He will be the head of Yisrael.
Rashi: Some say that it refers to Melech ha'Mashi'ach. However, the entire Parashah discusses Bayis Sheni.
Malbim: He is called Tzemach because his planting is not finished. He is prepared to sprout. He is called Zerubavel because he was seeded in Bavel. I.e. he is like a seed in the ground. It will sprout in the future.
What is the meaning of "umi'Tachtav Yitzmach"?
Rashi: Will come from the royal seed.
Radak: His level will come to him from his place - Yerushalayim.
Malbim: He himself will not sprout. Rather, from his seed will sprout one who "Yivneh Heichal Hashem" (13) - the final Bayis in the end of days.
