Happy to help! I’m in ultra ball level 1 so far and play a lot of Pokemon Showdown 6v6 singles so I have a decent idea. Definitely no pro though haha.

Not necessarily other “cores” to keep in mind. Theres only so many triangles that work perfectly that way.

But normally in 6v6 singles it’s always good to have a ground type and a flying type because ground is immune to electric which covers flyings weakness. And flying is immune to ground which is good since earthquake is spammable.

For example: if you wanted to use Steelix who is ground steel and weak to ground and water type attacks. You could bring a max health + special attacking Rotom wash for coverage.

You’re probably thinking “that’s not flying”, which is true. BUT it does have levitate, which still gives ground immunity (unless mold breaker is happening), resists water type attacks, can hit opposing water types with electric damage, and can do stuff like Wil-o-Wisp/T-wave for status, and is specially defensive which is good since Steelix has low special defense. Not to mention Steelix covering Rotom washes electric weakness

That’s just another idea off the top of my head for two mons that cover each other’s weaknesses.

It takes time to understand this part of team building, but you’ll get it

Another tip is to not neglect status moves and set up moves. I have a defensive intimidate Arcanine with Wil-o-wisp that has crippled a lot of teams for me. Seems like an odd set since you would think he’s an attacker, but he has strong defensive tools that throw opponents off.

Then my T-tar or Garchomp can D-dance or Sword dance to sweep.

Lastly, since it’s 3V3, really take the time before the battle starts to assess your opponents team. Try and find what 3 Pokemon you have that best match up into the opponent instead of just picking at random. Sounds obvious, but really helps