Question: If a girl over 9 gets bleeding for the first time, for less that 3 days and it is istehaaza, will she still be classed as baaligh (mature).

Answere: A girl becomes baaligh when her periods start (haydh), and not when any bleeding starts. The minimum period is 3 days and the maximum is 10. See nurul iydah page 38:

وأقل الحيض ثلاثة أيام وأوسطه خمسة وأكثره عشرة.
“The minimum period for haydh is 3, normally 5, and at most 10”

Since her bleeding is less than 3 days, it would be classed istihazaa (non-menstrual non-postnatal bleeding) and not haydh, so she would not be considered baaligha yet. (a/hafiz1/12)