Too many letters pose problems?

Do colleges view multiple recommendation letters from former teachers/mentors unfavourably?

I've found no evidence that points to this view.

College generally prefer at least two academic teacher references. A reference from a teacher/mentor leading extracurricular activities like art, music, volunteering, or athletics can often serve as the source of a good additional character/activity reference.

That said, admission offices receive thousands of applications so limiting the length of your application is also recommended. I would suggest two academic references and an extracurricular reference (if particularly strong).