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.
*Sigh* You're so willing to just feed someone the information without encouraging them to have a go at it. I can't fault your helpful attitude; it's refreshing to have someone who's nice and capable helping out. However, I bet that Bloop would be able to figure out something that works with a little bit of exploration on his part and helpful prodding on ours.