I believe that will be three new classes because that's what the game will need to function properly at Stormblood launch as things stand now. I already explained why in great detail, but I cannot seem to reiterate enough just how horrible the DF, integral to the experience of every single player past level 15, would be detrimentally affected without a proper trifecta choice for each role on release. It would be absolutely awful.
RDM being confirmed as DPS and not a healer was all I needed to know that SAM was gonna be stuck tanking and whatever 3rd job they get would be healing. I honestly don't think SE is dumb enough to do otherwise, unless they are putting in work changing the fundamental structure of the game with something like class specs a la WoW, which isn't something I'd be betting on anytime soon over the afformentioned.