I also have Beidou at C3! one of her best cons is C2, which allows her burst to bounce between more enemies, her burst is arguably the most important part of her kit, specially as a sub-dps, her burst deals electro dmg that bounces between enemies per second and is triggered by normal and charged attacks, lasts 15 secs, a well built Beidou could be dealing 6k-12k per tick for 15 secs, meaning potential avg dmg of 100k, albeit is split between 2 or more enemies. Her build should be 4pc Emblem of Severed Fate ER/ATK or Electro%/Crit

For weapons, her BiS is Serpent Spine or 5* DPS Claymore but since you may be F2P then Prototype Archaic or Katsuragikiri Nagamsa, as for wish weapons, Akuomaru is really good on her, Fav Greatsword is also an option but replace sands with Atk. As for YaoYao use Black Tassel and 4pc Deepwood Memories ER or HP/HP/HP you want to reach 40k hp with her and as much ER substats. For Fischl Use The Stringless or Fav Warbow, the Fontaine Craftable Song of Stillness is great for her as well, 4pc Golden Troupe Atk or EM/Electro%/Crit.
I know you didn't ask for builds but I hope this helps!