I get really tired at around 3pm every day. I eat well, exercise regularly and sleep well too. Why am I so tired at that specific time?

2 things. 1. Possible sleep apnea 2. Low blood sugar see your family doctor an get evaluated for these things. Keep up the other things and good luck. May only need nutritional assessment and recommendations.
Possibly normal . Assuming you don't have sleep apnea, other medical problems or take some meds that cause drowsiness this could be due to what you're eating for lunch or a normal lull we all feel in the early afternoon as part of our circadian rhythm and sleep/wake drive.