There is no third region. However, there are extra places you can visit by completing specific task,
NO!
You cannot go to the Sinnoh region in "Pokémon HeartGold" and "Pokémon SoulSilver."
Not in Heartgold/Soulsilver.
No, there are only 2 regions in Pokémon HeartGold, the Kanto region and the Johto region.
no
You cannot get to the Sinnoh region in Pokémon HeartGold, you can only travel around the Johto and Kanto regions. No additional regions are available in HeartGold.
there is not a new region its in johto
You can't get to the Hoenn region in HeartGold or SoulSilver. However, Latias can be caught in HeartGold, which is a legendary from Hoenn.
no
That area is not in that region.
Johto.
Johto.