What do you want as "Third hand"? if you want something like switchable hotbar, it should be possible...
Uh, you mean "displaying a map"? Well, as displaying items on the right corner is only possible with some achievement or recipe displaying tricks, I can definitely confirm that displaying a map on a corner is impossible for now.
but you could summon a map-filled item frame somewhere at the player so you could look at the map (would not be that beautiful lol)
The only thing you can is using a minimap until it will be possible.