There's no such guy in Sandgem Town that has 3 Master Balls.
Yes you can.
sorry but it isn't instant but in soleceon the guy next to the Pokemon center go in there SAVE IT talk to him he will ask you to bring him a Pokemon if it is a legendary catch it and sae it but you don't hae to cause there is 99% chance of it being three master balls but you might be that 1% so i would save then talk to him he might bring you three master balls but if not start again. but if it is a normal Pokemon it is rare for him to bring you 3 master balls. hope this has helped :-)(-:
you get three master balls and look for them allover then cach em..all lol
I would advise, putting them asleep, then false swiping them til they have 1 hp, then spamming Ultra Ball, if you have plenty of master balls, use those.
no you can not
trade from emerald
There is no Mew 3.
You cannot catch Registeel in Pokemon LeafGreen. This is due to FireRed and LeafGreen being able to trade with Ruby and Sapphire. Pokemon Ruby and Sapphire have the three legendary golem Pokemon including Registeel.
There is only ever one Master ball in any individual Pokemon game. However, if you have two copies and two Gameboys/DS's/whatever, you can have a pPokeon hold it while trading to get another Master ball from the other game.
AT THE BEGINNING OF THE GAME!!!!! or one,two, or three island
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.