what version of war3? It's different for each version:
Warcraft 3 Frozen Throne or Warcraft 3 XP? Change mp_xpmultiplier. The lower the number the easier it is to gain XP, I usually set mine to 0.7. (if it's frozen throne you can edit this in your war3ft.cfg file, if it's warcraft 3 XP just put it in your server.cfg file)
Warcraft 3 Expansion: look in war3x.cfg and set war3x_multiplier to a number, the higher the number the more XP you gain per kill/objective.
Ultimate WC3 (I think): Look in the war3ft.cfg file and change the mp_xpmultipler, similar to the frozen throne way