In the name of Allah. All praise is for Allah and may blessings be sent to his Messenger, Muhammad Sallallahu alaihi wasallam.

As you have mentioned, the minimum requirement for the necessary recitation in Salaah is three verses or one verse equivalent to three. However, the minimum requirement for the amount of letters necessary is, according to one opinion, eighteen. Another opinion is, that there should be at least thirty letters. the second opinion is the more safer one to follow.
(Shami, p. 149, v. 2, Beirut, Lebanon)

Answer Approved by Shaikhul Hadeeth Mufti Farooq