The only "operative" of Section 31 that we know of is Sloane. The other two "operatives" that were seen were in the holodeck with Sloane when Bashir penetrated the ruse. They could have been fictional, too, to put Bashir off of attacking Sloane.
Nope. Agents of Section 31 appeared in the ENT episodes "Affliction," "Divergence," and "Terra Prime," set over 200 years before DSN. Just as Sloan said in "Inquisition" about Section 31 being centuries old.
Meanwhile, there's also Julian and O'Brien literally entering and reading Sloan's mind, and finding his knowledge about Section 31's operations on various worlds in the episode "Extreme Measures." They literally read his mind and saw that Section 31 exists.
Section 31 is a genuine criminal conspiracy, a cabal within the Federation government.