I was one of only ten kids in my grade of 270 to be placed in the “Advanced Math Track.” I have completed Adv Alg 2, AP Precal, and I’m almost done with AP BC Calculus. Next year I’m going to take Calculus 3 and Linear Algebra with only 5 other seniors. Do you think this will give me an edge for colleges especially if I’m going into a STEM major? Or is it actually very common?
You will no shortage of students taking math classes as advanced as and even more advanced than calculus 3, especially in the applicant pools for competitive schools. That doesn’t mean that it won’t look fantastic on an application though 
depends where you’re applying. You’d be among about 250 students among the admitted to Harvard, in the middle 50 at Harvey Mudd, and way in the stratosphere at some public universities.
It does look good but not everything depends on it. How well-balanced your schedule has been - do you have 4 years of English, social science, and science, plus foreign language up to level 4 or AP? Do you have at least 5 honors or AP classes per year? Will you have 4-8 Ap classes by the end of HS if your school offers them? What extracurriculars do you have? etc