2 things there is more than 3 legendary Pokemon in the game and nothing is "unlocked" in the game if that is what you are asking
The legendary of Pokemon Black ( Reshiram )
Yes, there is if you bring the three legendary birds Zaptos ,Moltress and Articuno and ,bring it to 4th F the legendary Pokemon appearing is Lugia which can be captured! Source: Tried in white 2 and black 2
You fight with them nothing special really happens
If you havent beaten the Pokemon league then level them up a bit then defeat the Pokemon league
You cant its not possible to get them Melovepeas: ^that is incorrect. You cannot "find" the Three Lakes in Pokemon Black, as they're the legendary trio of the Sinnoh Region. It is possilbe, however, to migrate them from Pokemon Diamond/Pearl/Platinum to Black.
you don't have to to anything with the orbs. They only give you the orbs so if you transfer those legendary Pokemon to Pokemon black or white you can give those legendary Pokemon the orbs.
You have to defeat the elite four then you can catch the legendary
No legendary Pokemon in history can breed. GENDER OR NOT!
Groudon, Kyogre, and Rayquaza. Groudon is the fire legendary pokemon, Kyogre is the water legendary pokemon, and Rayquaza is the air legendary pokemon.
Zekrom in Pokemon White and Resharim in Pokemon Black.
Reshiram
No.