For Raquazza, you have to complete the main game and then go to sky tower. It's the same for Groudon. For Kyogre, I don't really know. I only beat the game and got legendaries.
Moltres, Zapdos, Articuno, Groudon, Kyogre, Rayquaza, Deoxys, Mew, Celebi, and a few others.
Like Kyogre it has a 50% chance(or maybe you "cant" recruit it)
Well, you can be any Pokemon you recruit. There are also many legendary Pokemon such as mewto, mew, suicune, raiku, entei, raquaza, groudon. kyogre, zapdos, moltres, arcticuno and many more.
groudon
Beat Raquazza.
kyogre
after you defeat groudon
Kyogre is a Water Type Pokemon. Water Type Pokemon are weak against Electric and Grass Types.
I believe if you look at red rescue team it is the same.
Groudon is located in Magma Cavern. He has a -10 chance of recruiting.
You may recruit Groudon after the end credits of the game.
You can get volcanic pit after recruiting Groudon.