I think this should work (hope you understand it):
Events
Unit - A unit Dies
Conditions
(Unit-type of (Dying unit)) Equal to [Your Unit]
Actions
Unit Group - Pick every unit in (Units within 512.00 of (Position of (Dying unit)) matching ((((Matching unit) is A Hero) Equal to True) and ((Owner of (Matching unit)) Equal to (Owner of (Killing unit))))) and do (Actions)
Loop - Actions
Hero - Add 3 experience to (Picked unit), Show level-up graphics
This trigger will add 3 experience to all heros in a region around the killed unit that belong to the player whos hero killed the unit. If you wanna have allied players also gain exp, tell me.