since account 1 and two are linked.. they have to be withing the same level.. of each other.. if your boosting 2 and 3 wtih your level 70 on account 1 then 2 wont get the xp from mobs..

read. before you do. there's ALOT of info already in this forums.

if you start 3 new toons.. all level 1 on all three accounts.. then yes.. all 3 will get all the bonus of xp.