Most Pokemon trainers may say it is impossible to get all of the starters, but actually it isn't. There are two ways. The first is through action replay. Here is the second:
All you need is another DS (borrow a relatives or a friends) and another version of a Pokemon Game (Pearl, Diamond, Platinum, HeartGold, SoulSilver, Black or White are the only ones you can use). First, you choose your first starter on one version. Then go the the Red Pokemon Center and trade it over to the other game. Then hit the erase your game, and get the next starter. Trade it. The erase. And get the last starter and KEEP it, you won't need to trade.
Now, go catch 2 wild Pokemon. It doesn't matter what level or what Pokemon it is. trade the two over to the other game and get back to the two starters.
(TIP: This cheat will also work with other games that aren't in the Unovs region, such as the games listed which feature the Sinnoh region, Johto region, and the Unova region.)
they are in the unova region
You can't, but you can get them all through the global trading system or gts.
To obtain all three starters you need to trade with someone.
All 3 Unova and Johto starters can be found in Pokémon Sun & Moon!
you can get all the pokemon in the Unova region in pokemon black and white, in the other games you all the pokemon in sinnoh, hoenn, kanto and johto. Unova came out in 2010 and shortly after that they thought of a game / 2 involving Unova region pokemon.
there are many great starters. in water starters have only two weaknesses, fire starters have four and grass starters have six but that does not make one of them better. I don't think there is any best starter.
Trading.
in my heart
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
Kanto: squirtle,bulbasaur,charmander Johto: totodile,chickorita,cyndaquil Hoenn: mudkip,treeko,torchic Sinnoh: piplup,turtwig,chimchar Unova: Oshawott,snivy,tepig These are all the starters and these are the ones i started with in each game: Charmander, Totodile, Treeko, Chimchar, Tepig
You can ONLY go to the Unova region without cheats and hacks. Although after you beat the Elite Four, new routes are unlocked with Pokemon from all of the other regions, though this is still in Unova.
It is impossible unless you hack