So the question remains, what happened to the other two? My tin foil hat guess, ::)or maybe not so tin foil, is the Council felt threatened by the power of the island and the leyline that runs under it. Considering what they went through with Kemmler, whom trying to tap into the power of the leyline and became totally corrupted and dangerous by it, decided to bump off the two newer Wardens. Considering that Harry seemed to be set up to get the boot from the Council, and the Blackstaff possible orders to do him in.. It is my guess that Eb killed off the other two Wardens on Council orders..
Jim has indicated the job is pretty inherently dangerous.
Prior wardens could have died trying to capture various BigBads.
They may have died because a succesfully-captured BigBad had cults of followers and/or Lieutenant-MediumBads whom the Warden didn't adequately prepare for.
There's plenty of *other* Supernatural forces in the world, not just the WC, who might have taken exception to something the Warden did, or was expected to do. From what we have seen I expect the Archive could have taken out a Warden... and likely at least
some of the other individual signatories.