I already know where Rayquaza, Groudon and Kyogre are, but I can't catch..

You can find it on the Lilycove city on the t. Aquas team

First of all, I think there isn't any Master Ball in Mirage Island.

Secondly, don't use Master Balls on Grou/Kyo/Ray. Reserve that for Latios/Latias.

Thirdly, the only ways to get more Master Balls is to win the lottery in Lilycove City, trade or cheat.

^_^

Eventully you will catch them it just's takes some time and a lot of ultra balls!
The other master ball is in the aqua hideout.
And catching Latios is at random.

Rayquaza- At the top of The Sky Pillar.
Groudon- It depends on the Location Specified by the Weather Institute.
Kyogre- It depends on the Location Specified by the Weather Institute.

Latios appears in random places. Every time you go into a door, cave, or other Entranceway, it changes. It also changes when you use Fly. (Not in battle!)

You can get a master ball by winning the Pokemon lottery three times, or in the enemy hide-out.

Southern Island.

1. Well, a master ball is found in Team Aqua's hideout just on the outskirts of Lilycove City. It's located deep inside it. You can obtain another by matching all 5 numbers of your ID# with the number on the ticket you chose in the Lilycove Lottery Corner thing. It's hard to match, so be patient. By the way, there is no master ball on Mirage Island. Whoever told you that is a total lyer.
2. He's found randomly all aroun Hoenn. Be patient, because you run into him with luck or by coincidence. But, when you do run into him, track him down if you want him. By the way, save the master ball for catching Latios. Rayquaza, Kyogre, and Groudon should be easy to catch with an ultra ball.

Latios is running around the areas try searching on water for him. You can catch the Pokemon without a masterball.

Use a bunch of timer balls and ultra balls when you encounter them. Have them sleep, and have low HP. Go to the ROUTE where the daycare center is, have two Pokemon battles in the grass at that route, if you did not encounter latios, go into the daycare center and back into the grass to try again. If you encounter Latios and it flees, then go to the daycare again, but this time, when you get out, check your pokedex Latios entry to see where it was lasted spoted. If it's not in that route, then try again. I keep Latios from fleeing, you can use Mean Look(recommanded), a master ball, or have it sleep.

Use a action replay to get more master balls,or a gameshark.

Latios can be found any wheres,but in canves and on the islands,Latios can be found on the water,tall grass,short grass.

Hope I help

Hi
For a easy way to catch rayquaza, refer to my answer lower down on the page, cause I don't really want to repeat myself. I wouldn't use a master ball on him or Kyogre or Groudon, maybe save it for Latios. The level 70 legenderies are easy enough to catch, as long as you have plenty of ulra balls, timer balls, revives, hyper potions and a Pokemon who knows false swipe and something to paralize or put things to sleep. The secret is that after being in the battle for ages, as I said before, break out the timer balls and they will be of great use. Personally, except for Latias I have caught all my leganderies in timer balls. As for the Masterball, I don't think there even is one on Mirage island, as far as I know you can only get a rare liechi berry and wild wynauts, and the masterball is just another rumour. You can get the original Master ball in team magma's/aqua's hideout, and if your lucky, another one by winning first prize at the lottery corner.
As for Latios, it's hard to find him for the first time because he roams around anywhere and it's random where he'll be, so your best chance is just to walk around in the grass for ages until you encounter him. A tip is try not to fly anywhere, because his location will reset. Hope I helped, good luck!

In team Aqua's hide out, in one of the room's you will find 2 voltorbs, a nugget and a Masterball. Also a hint to catch Kyogre, use a Shedinja because it can't hurt it. Then have heaps of timer balls and ultra balls with you. When Kyogre starts hurting itself with struggle, start using timer balls and ultra balls. Hope I Helped!

You can find another masterball by winning the lottery in lilycove city but you're id must match what the announcer says.

Latios can be found in the wild just like any other Pokemon, but randomly. It will change it's location once you leave a city or change a route. I sugest this way to get Latios:

-Get an Acro Bike
-Buy lots of Repels,super repels,etc..
-Head over to the bike route on Route 110
-Keep coming in and out of the first shack leading into the bike route until you see Latios on your map
-Head over to the nearest shade of grass and use repel
-Then hop on your Acro bike (using B) about 20 times

This may take around 10-15 minutes but it's worth it. Latios is at Lv 40 once you encounter it. It will flee from battle immediately unless you trap it with the moves BLOCK,MEAN LOOK, AND SPIDER WEB. Or you can get a Wobuffet (because it has the special ability to not let the wild Pokemon flee).

AAANNNDD... BADABOOM, The Latios is yours!

By the way I have a question..
HOW DO YOU GET JIRACHI IN POKEMON EMERALD?

You can find another masterball by winning the lottery in lilycove city but you're id must match what the announcer says.

Latios can be found in the wild just like any other Pokemon, but randomly. It will change it's location once you leave a city or change a route. I sugest this way to get Latios:

-Get an Acro Bike
-Buy lots of Repels,super repels,etc..
-Head over to the bike route on Route 110
-Keep coming in and out of the first shack leading into the bike route until you see Latios on your map
-Head over to the nearest shade of grass and use repel
-Then hop on your Acro bike (using B) about 20 times

This may take around 10-15 minutes but it's worth it. Latios is at Lv 40 once you encounter it. It will flee from battle immediately unless you trap it with the moves BLOCK,MEAN LOOK, AND SPIDER WEB. Or you can get a Wobuffet (because it has the special ability to not let the wild Pokemon flee).

AAANNNDD... BADABOOM, The Latios is yours!

By the way I have a question..
HOW DO YOU GET JIRACHI IN POKEMON EMERALD?