xp will divide by 3
on 2 person leech case
imagine like, a mob gives 1000 xp
when someone kills it and its on even share, he/she gets 50% of that xp so, 500 xp go to the person killing and 500 xp for the person leeching (unless it have cards that modify that, dracs for example)
on a 3 person case
the same mob that give 1000 xp
who kills gets 33% of that xp (330 xp) and the other 66% gets divided by both leechers (33% for one and 33% for another)