Medellín is 7 hours behind Athens
Best overlap: 3:00 PM to 6:00 PM in Athens is 8:00 AM to 11:00 AM in Medellín.
Aim for 3:00 PM to 6:00 PM Athens time — that's 8:00 AM to 11:00 AM over in Medellín. With 3 hours of overlap, 3:00 PM in Athens (8:00 AM in Medellín) is a comfortable, repeatable slot for both sides. Remember that Athens shifts for daylight saving and Medellín doesn't, so recalculate the overlap after each clock change.