I thought I put the video to that youtuber but apparently not...
https://www.youtube.com/@airmong
He has a lot of videos of 3 turns, but as I've said, he's a superwhale so everything is NP5. That said if one of your Servants is NP5 maybe you can replicate what he does.
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As for my setup, I use Caster Cu (with damage CE), double Koyan and Tiamat (you can replace her with whatever you want, recommended Caster Gil or Helena here, I don't use them because I already have them at Bond 10):
1st turn: Caster Cu 1st and 3rd, use all the skills of one Koyan, swap her for Tiamat/Caster Gil/Helena/any 50% NP charger. Use the new Servant skills to get them to 100% charge. Then do their NP first then chain Cu Caster.
The rest of the turns are basically the usual stuff with double Koyan + Caster Cu combos.
Do keep in mind that I have a level 100 Caster Cu at NP5 and 2K Fou, so I can't promise this setup works on a weaker version with no grails (for that check Plushie's video).