@ Pavious, in S01E08: "Dax", it was stated Jadzia Idaris graduated Star Fleet academy BEFORE being joined and perused being joined only after she graduated; well maybe pursued is too strong a word as one would suggest her advanced studies in science was part of the journey to being joined. Memory Beta is where I am pulling the fact she was stationed on Trill before DS9 but that information in of itself is pulling from the novelized version of Emissary, so take it as you will.
The odd thing is, even in some situations where Kira and Worf are both present on the station, Dax is clearly the one in charge. An example that comes to mind (though Kira has mitigated circumstances for not being charge) is S05E10: "Rapture". Dax clearly pulls rank over Worf. That is after S04E18: "Rules of Engagement" where Worf gets the "talking down to" and eons before S06E16: "Change of Heart" where he sacrifices the Cardassian informant to save Dax.
Frankly this whole mess would have been easier to explain if, like Roboturner913 said, if they just would have stuck Jadzia in a red shirt and explained it away as "DS9 is a busy place and Sisko needs 3 XOs" >.<
Frankly, with as much as was going on and as much as he was in charge of, this isn't really unreasonable. Like I said earlier, its amazing how much Dax, Worf and Kira stepped on each others toes without actually ever getting in arguments. One could take the high road on that and say all three were really mature officers that really understood when who should be taking charge and when they should let them and not have a chip on their shoulder for it.