they roam around in the wild after you have done everything on the islands but they run away
just go to the pet shop and buy all of them.
The legendary dog you find the dog that has a type advantage over your starter.
moltres
You would have had to chosen Charizard as your starter Pokemon. Whichever legendary dog appears depends on your starter. You will find your legendary dog randomly in the wild. (you must beat the elite four first)
No
Entei, Raikou and Suicine are often referred to as the Legendary Beasts or Legendary Dog pokemon. They aren't in Diamond, so you'll have to trade or migrate them. You can do this from the following games: FireRed/LeafGreen, HeartGold/SoulSilver and OmegaRuby/AlphaSapphire.
Raikou is a legendary dog Pokemon. It is not in any specific location in Pokemon Fire Red. Instead, you can find it roaming Sinnoh.
the 3 birds
Moltres is a legendary Fire and Flying Pokemon. Part of the original trio of legendary birds from the first games, it can be encountered in Pokemon FireRed in Mt. Ember.
(Excluding legendary pokemon) It should be dragonite...
the fire Pokemon that professor oak give ( charmander ) and also growthlite ( a dog Pokemon ) Forgive me if spell wrong
yes but it depends on who and what you are fighting