Hi I was just wondering how to get a master ball. I never picked it up..

You have to win the lottery to get a masterball

You can't find the three dogs in leafgreen I think

Hi first you wasted your chance by not getting it from the hideout BUT you can get it by winning the first prize at the Pokemon lottery in lilicove city(do so by having all 5 numbers matched)
But this is pure luck although I can suggest you have as much as possible Pokemon in your party/pc with DIFEERENT ID numbers.
Secondly, you DO NOT need a masterball to catch latias, although everybody says you have to.
I'll teach you my method-i catched her with it.
Firstly, have a swellow dat knows endeavor.
Have it at level 40 or higher and equip it with the quick claw(found in the Pokemon school, rustboro) so that is speed is higher than latias's
Then, you have to bring it's hp down to 1. You can do this by poisoning it, walking around with it and when it has only 1 hp left, use antidote.
I must ask you-have you already spotted latias? You have to do so or otherwise, this trick won't work.
You also must have a wobbufet(or wynaut) at a level as high as possible (so it can take a few attacks from latias). Put swellow first in your party
Now, go to the weather institute and check if latias is in this route. If not, go in the institute and check again.
When she's in the route, go looking for her in the tall grass.
When you encountered her, use endeavor.
Latias will now have only 1 hp.
Search for him again, but this time, put wobbufet first.
Encounter, and now, just keep using ultra balls(because of it's 1 hp, you won't even have to throw in a status problem)
I catched her from the first shot, but it might take a while.
And voila, you catched latias without having to use a masterball
Sorry for the long writing :(
NOTE: this also works for rayquaza, using endeavor and stuff, actually for ALL legendary's

Then, you also asked if you could catch the legendary dogs in leafgreen.
Answer is that you can, but only one of them depending on your starter.
Charmander-->suicune
Bulbasaur-->entei
Squirtle-->raikou
They will appear random in kanto(use trick listed before!!)

I really hope i've helped, and that is was all worth writng
Shadow lugia

To get the Master Ball, you need to win the lottery at the Lilycove Department Store.

You can't catch all three dogs in one file, there is only one dog and that depends on who your starter is:
Bulbasaur - Entei
Charmander - Suicune
Squirtle - Raikou

You would have to go to a Nintendo Event to get to eight and nine island.
*Note that Nintendo Events don't appear anymore unfortunately.