Left turn signal not working properly

My left turn signal will start blinking normally when used, then after 3 seconds it begins blinking rapid like a bulb is burnt out. I have checked all 3 bulbs (front, rear, and mirror) and none are burnt out. I was wondering if anyone could help me find which fuse or relay commands the Left turn signal so that I can check those before replacing bulbs. Again, all bulbs appear to function properly.