An excerpt from Game Informer interview wit Hiromasa Shikata at Nintendo regarding Triforce Heroes on 3DS…
GI: Where does Triforce Heroes take place in the Zelda timeline?
HS: That’s a tough question. The Zelda timeline is quite complicated if you look at the history of Zelda I think you can see there are three branches. I can’t really designate which one of those branches we’re looking at, but as far as the design itself, it really is Link Between Worlds. But it’s not – as far as a timeframe – before or after. We haven’t really settled on or said that.
GI: Does it take place in the same universe or world as A Link Between Worlds?
HS: Again, with the history of Zelda we have these three parallel worlds. I can’t say which one it’s in at this point.