TOXAPEX - So your Toxapex is VERY gimmicky and doesn't work very well in a competitive environment. (Like Battle Tree). Here's some ways to improve your Toxapex. 1.) Give it Regenerator. Toxapex is very slow and can sometimes just barely survive a hit and not be able to use Recover fast enough. This is a perfect opportunity to switch and get a Regenerator recovery and save Toxapex for later. 2.) Give it Icium Z. Your team is missing a Z Move and Z-Haze is very useful. This allows Toxapex to not be shut down as easily when exposed to Knock Off. I know you don't have Haze on Toxapex ATM, which brings me to 3.) Change his moveset to Scald, Haze, Recover, and Toxic Spikes. You could use Toxic or Baneful Bunker instead of Toxic Spikes but I prefer Toxic Spikes to badly poison opposing pokemon that try to switch in AFTER Toxapex faints. Unfortunately, Toxic Spikes can easily be removed by an opposing poison type, making Baneful Bunker would be better in some scenarios as it can scout out AND poison. (Just regular poison though, not badly poison). Your EVs are fine, but just as a minor suggestion give it 0 Attack Is to minimize confusion damage.
GOLISCOPOD - Goliscopod has incredible stats but a terrible ability, making him not very viable in any tier besides RU, and AAA. Also, adding him would make your team 2/3 water type and have all 3 pokemon neutrally effected by electric, which brings me to a few proposals as replacements for your Goliscopod.
- Landorus-Therian
- Mimikyu
- Durant
Landorus-Therian @ Assault Vest
Ability: Intimidate
Adamant Nature
EVs: 212 HP/44 Atk/4 Def/220 SpD/28 Spe
- Earthquake
- Rock Tomb
- U-Turn
- Knock Off
Description: A complete and utter tank. The main problem with Landorus-T is that any pokemon with BoltBeam coverage (that's Thunderbolt and Ice Beam) can completely trash your team. However, Landorus-T is a great switch-in for electric types and provides niche utility in Knock Off and Rock Tomb.
Mimikyu @ Focus Sash
Ability: Disguise
Adamant Nature
EVs: 252 Atk/252 Spe/4 SpD
- Shadow Sneak
- Shadow Claw
- Play Rough
- Swords Dance
Description: While very easily shut down by Taunt and Cloyster, Mimikyu thrives in the 3v3 format as it enjoys relatively decent tankiness, priority, and most of all access to SD and Disguise, which is totally broken. You could also run Mimikyunium Z and keep Toxapex holding the Black Sludge to free up a Z-Move spot. Another option is Life Orb, as it offers nice damage, and Mimikyu is bulky enough to USUALLY tank a hit and be able to set up 2 SDs anyways. You could also run Jolly over Adamant, but Mimikyu generally spams Shadow Sneak anyways. Another option would be running Wood Hammer over Shadow Claw for more coverage, but grass offers minimal coverage anyways, so I prefer Shadow Claw to potentially OHKO tanks after setting up an SD.
Durant @ Choice Scarf
Ability: Truant
Jolly Nature
EVs: 252 Spe/252 Atk/4 Def
- Iron Head
- U-Turn
- Entrainment
- Rock Slide
Description: None of these moves really matter except for Entrainment. The idea of this set is to change your opponent's ability to Truant and then switch into Salamence-Mega. Then, you can reliably set up Dragon Dances and probably sweep with Mega Salamence from there. If you want to use this strategy you might want to replace Salamence-Mega's Roost with Protect to use DD on every Truant turn and Protect every turn they can actually attack. This set is rather unconventional and is shut down by Prankster Taunt and the opponent just switching out.
In all, I'd suggest Mimikyu as your replacement to Goliscopod, for reasons I stated on its description.
SALAMENCE-MEGA - Your Mega Salamence is mostly fine, but I can spot a few minor issues that could potentially result in an unfortunate loss. First of all, Dragon Claw is prefered over Roost for STAB. Roost + 44 HP EVs is only really useful in Ubers so that Salamence-Mega can tank an HP Ice from a Landorus-T if it outspeeds. As such, I would change Salamences's EVs to 252 Atk/252 Spe/4 Def to outspeed more, and Double-Edge instead of Return for maximum damage output.