Grades in a PhD program

I am a math grad student who has completed the qualifying exams within my first two years. Lately, I have been taking a few non-core courses, which allows me to put most of my time on my own research. In one of these non-core courses, I received a low grade: B-. I certainly have the a good Cumulative Grade Point Average: 3.9 -- well above what is required for "normal progress".

Question: How important/serious is getting a grade below 3.0 in this one course at this point in a PhD program? Any feedback is much appreciated.