catch a budew at route 204. it already knows grass moves. once it evolves to roselia (for me around lv. 24)
You didn't pick Turtwig?
Then if you don't want to raise a Grass type, (Budew might be a little underleveled at this point if you don't want to train) just teach Grass Knot to whichever starter you picked. Both Empoleon and Infernape can learn Grass Knot IIRC.
Do you mean the third gym (at Veilstone) or the fourth gym (at Pastoria)? I don't see why you need any Grass type Pokemon in the third gym...
Teach one of your Pokemon Grass Knot and call it a day.