trade them from Ruby, Emerald, or Sapphire, or... trade them from a friend, or... get them from the GTS.
hoenn is the region Pokemon ruby, sapphire, and emerald take place in. if you bought one of those three games, you found the hoenn region.
Torchic, Mudkip and Treecko.
You cannot get to Hoenn in Heartgold as it is a remake of the generation 2 game, Pokemon Gold and the only regions available in Heartgold are Johto and Kanto. You can however, obtain one of the three Hoenn starters by visiting a man from Hoenn at the Silph Co. building in Saffron City.
One of the three Hoenn Starters. Treecko, Torchic, or Mudkip.
It is one of the three starter pokemon you choose from at the very beginning of the game. if you didn't choose it, there is no other way to get it unless you ask for it on the GTS
You can get them by visiting Steven Stone at Silph Co. after you defeated Red.
You have to have three different games for that to happen.
You can get three starters from different areas by completing the game (one at the beginning from johto, and after defeating red one from kanto and one from hoenn) but unless you trade or cheat there is no way to get all three starters from any one area.
When you first start the game you will get a starter Pokemon, then you beat all 16 gym leaders and then you need to beat a man named red at the top of MT. Moon and go back to professer Oak and he will give you a choice of 3 more Pokemon then go to Hoenn and get a choice of three more Pokemon
He will give you one of the three Hoenn starters, and will also notify you of Latios' (Or LATIAS) arrival to the region.
There's no such thing as Mew Three.
By Ruby I'm assuming you mean the Hoenn starters, then yes, you can get one of three of them in HeartGold/SoulSilver