You can, if you really want, already pay for Black Ops 3. It's currently £39.99 on Steam. If you really really want, you can also buy the season pass. It's £34.99.
So, if you really, really can't wait, you can buy a game that isn't out for six months (and has barely been seen), along with all of its unannounced DLC, for just shy of £75. Although, you can save a bit by going for the Digital Deluxe Edition, which bundles it all together for the low, low, actually not that low, price of £69.99.
It seems like only this week that we were spitting tea at Batman Arkham Knight's £33 season pass (because it was this week), but at least COD is following suite, as Advanced Warfare's season pass was also £34.99. Like Batman, Black Ops 3's actual DLC content is mysteriously undefined, with it simply stating that this provides "access to four exciting DLC packs". Although, given COD's fairly predictable template it's almost certainly four multiplayer map packs. And, for those that will buy them all anyway, probably money saved.