Well, a couple possibilities come to mind. First, as others have mentioned, they retreated before the combat ended, so they might not get much xp. Second, they really just created a 'trap,' albeit a trap big enough to wipe clean several hexes - I dunno if you get xp for killing something with a trap you made, since it's kinda indirect. Third, the 3-way split will reduce the xp gain significantly. Fourth, I know a lot of games in which the highest level char used in a battle determines the amount of xp gained (so the total xp gained is based off the highest level character's rate of xp gain). Fifth, it could be that the linked casters, as a single entity, gained xp, so the xp went towards the linked form and did nothing for the separate casters. Sixth, it might just be that their combined levels count towards the link (see V. with the soul-splice in OOTS); in this case, they might just be too high level overall to gain much in the ways of xp. If a master-class is, say, lvl 10, and there's at least one master-class in the link (wanda) then the combined levels of the link would probably be 20 or higher - so little if any xp would be gained.
(Hopefully at least some of this is coherent/logical/possible...)