I used monistat 3 for a yeast infection, i've finished the three days of medicine, now i have some yellow discharge, wondering what that could mean/if that's common?

Infection? STD?: Sounds like you didn't have a yeast infection, or yeast plus something else. Vaginal yeast infections usually cause scant white discharge, or none at all; the main symptom is itching and irritation, not discharge. Colored discharge usually indicates other kinds of infections, especially STDs. Assuming you are sexually active and possibly at risk see a doctor soon for exam and testing. Good luck!