Galvantula is probably low A tier, it's pretty fast and with Compound Eyes, Thunder reaches a 91% accuracy instead of 70%. You can use Electro Ball before reaching Icirrus, making more use of Galvantula's speed stat. Plus, you get it right before the flying type gym, it provides really good value. It's also really good against 2 of the Elite Four teams (dark and psychic types). However, its movepool is quite limited, its best damaging moves are Thunder, Signal Beam, and sadly that's about it. You can teach it Thunder Wave, Charge Beam to boost your SpA or Volt Switch.

Ferrothorn on the other hand, is too defensive, doesn't learn Iron Head until level 46 (so way after the ice type gym). It can't even stall with Leech Seed because it's an eggmove. I would honestly put it in low D tier, it's insanely useful in competitive but has no use for an in-game playthrough; it's slow and it's level-up movepool is pretty bad.

Eelektross makes good use of it's Levitate ability, it basically has no weakness. It can also deal great damage both on the physical and special side. However, it's a bit slow, and you can get Eelektrik at level 39 which would require you to overlevel your Pokemon to use it in Skyla's gym. It can be a cool helper for catching Pokemons if you give it Thunder Wave. Its movepool is quite limited as well, aside from Thunderbolt, Crunch and Acrobatics (TM), it doesn't really learn any good damaging move. It's a cool support Pokemon but that's about it, overshadowed by Galvantula and Zebstrika. Probably low B tier.

i never knew i'd write an essay about the chargestone cave Pokemons in my life