what is the best grass water and fire pokemons that i can get in pokemon black ??
Other than starters, your best non-legendary options for a Grass/Fire/Water Pokemon in BW are IMO:
Leavanny - It may have a massive pool of weaknesses but learns some good attacks.
Whimsicott - Its Prankster ability is sweet cos it allows its status moves to strike before the opponent.
Lilligant - You can trade a Cottonee for a Petilil in Nacrene City if you talk to a girl there. It's a great Grass-type that can sweep things after setting up Quiver Dance.
Darmanitan - Great physical-based Fire-type that learns a few Fighting-type moves too.
Chandelure - Its sky-high Special Attack makes you want it, highly recommend it.
Jellicent - It's very bulky and has some good Special Attack backing up its high Special Defense and HP plus it learns some great special-based moves.
Seismitoad - It's one of the few Water-types to lack Ice moves but it does get Drain Punch and has really good stats all-around.
Swanna - Its 4x Electric weakness may not be fun but it makes a good Dive/Fly/Surf slave.
Also, the elemental monkeys (Simisage/Simisear/Simipour) are good too due to their TM diversity plus you can catch all 3 in Pinwheel Forest/Lostlorn Forest in shaking grass.