If you're Pokemon Red/Blue, you have one of the 3 Kanto starters. Obviously if you have Squirtle or Bulbasaur, the first two Gym's aren't too bad while Charmander is at a severe disadvantage due to having normal and fire type moves(there are no Steel, Dark or future generation types or moves to call upon).
Best means of handling Brock are Mankey(level 9 Low Kick), Nidoran(Male or Female since they both learn Double Kick at level 12) or Bug-types like Butterfree/Beedril. Against Misty, Bulbasaur owns the day again while Squirtle has some resistance to water type moves(granted Bubblebeam still does sting). With Charmander, you're at Misty's mercy so other Pokemon must be used. Pikachu is a strong counter if you use Thunder Wave to paralyze it, as would Mankey's pure power(Karate Chop at level 18 does wonders thanks to its high critical hit). You can also utilize Oddish/Bellsprout to battle her if you persevere in training them.
If you're Pokemon Yellow(like me), Pikachu is your golden boy, hence Misty is a piece of cake if you really train Pikachu hard. He learns Thunderbolt naturally at level 26, although its possible to kick her butt with Thunder Wave and Thundershocking her to victory. Bulbasaur is another option if you figure out how to acquire it(no ditto glitch necessary), as would Oddish and Bellsprout. To counter Brock, merely use the Fighting type move counters I mentioned above or use Butterfree(Beedril is unavailable).
As I noted before, Sabrina is arguably the hardest Gym Leader, regardless of what Generation 1 version you have. Of the three Kanto starters, I find Charmander to have the best luck due to Slash's critical hit chance while also having decent speed to overcome the first strike ability of Sabrina's Psychic Pokemon. Bulbasaur obviously would be a suicide mission and Squirtle would be an ok option but he's not my first choice. In Pokemon Yellow's case, while Pikachu's speed is unmatched, he simply lacks the power to knock her Psychic types out.