It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
You go to professor britch and he gives the fire red starters
All Pokemon from Kanto, Johto, and Hoenn can be obtained in Emerald. Susrskit, Masquerain, Meditite, Medicham, Roselia, Zangoose, and Lunatone must be traded from Ruby or Sapphire. Other Pokemon like all of the starters must also be traded over.
You cannot get all 3 Starter Pokemon in Pokemon Emerald without trading for the other 2.
Cyndaquil and all other pre-fourth generation starters are unavailable in Pokemon pearl and diamond. You must transfer it from Pokemon Emerald. If you do not have Emerald, sign up on a Pokemon trading forum and find someone who is willing to trade you one.
Sorry, but the only way to get all the starters from Pokemon Crystal to Fire Red or to Pokemon Emerald is to get a gameshark and do it from there.
cheats
you get them by catching all 200 Pokemon
Trading.
You can first collect all of the Pokemon for the hoen dex in Pokemon emerald you will get 1 of the 3 Pokemon gold/silver/crystle series starters then use the get all of the starters trick to get them all then migrate them to Pokemon diamond or pearl.
cindaquil must be migrated from emerald version... in emerald, once you catch all the pokemon, the professor offers you one of three pokemon... you may pick from the johto starters (cindaquil, chikorita, and totodile)
You can only get one of the three Starters in Pokémon Emerald without use of cheating or trading. You can trade or use an action replay code to get all three (I don't know the code, so I can't help you there). Hope this helped.
To obtain all three starters you need to trade with someone.
Yes you have to trade because you can't get all the Pokemon alone.
Yes and no. You do need all three starters, and you need every legendary except for Deoxys, Celebi, and Jirachi.
beat the elite four get a gameshark and the code is:qw7av78cvbfhg4.=5544577 788955258858 4785dfvhrbdnqaz7 47852145ssdczskl;
You cannot possibly get all three. Only one without a hack.