Destroy: Yeah there was a big explosion, we all suffered some damage but are fine, GETH BUILT AN EMP BUNKER TO SURVIVE THE WAR AND IT WITHSTOOD THE BLAST (alternatively/in addition they have become pure AI no longer using physical geth platforms)
Control: Yeah the reapers just up and left, no one knows where they went or what happened to them man! (alternatively they all flew into a black hole and were crushed, that way reapers are dead in all three endings!)
Synthesis: People got green eyes for a while, who knows what happened, but turns out that the StarChild was just a damaged AI full of shit and people still fight!
Dont be surprised if man versus machine is not really a theme in the new entry, and they can throw in some token dialogue if you chose synthesis.
Similarly, no matter what you did to the Krogan, I suspect the outcode of their race for this sequel will be the same anyway, pre determinism is a great philosophy to have when you are writing a long running RPG series!
Also do not be shocked if there is some race or aspect of things that reflects (on a small scale) the possibility of you choosing synthesis, but it is only referenced as such if the green ending was taken. (if they even do ME legacy save imports)
So basically anything you did in the OT will be worthless? The endings should change the 'verse radically each, even the extended cut ones. EDIT assuming it is a sequel.
Anyone familiar with Bioware writing should not be surprised at them sweeping the consequences of the endings under the rug. Bioware writing is the perfect example of writing yourself out of corners through sheer laziness and ambivalence.