BT+0 btw, not even maxed yet because of lack of ingots lol
One of the most damage oppressive Lufenia+ especially when they start putting on their red aura and suck your brave.
When you consider the fact that these three do not even put a single debuff on the enemy (Garland's special effect is a debuff aura) and they are pretty much capping or near capping at the end of this fight says plenty about putting these three together. Jecht did not even need his own BT effect to reach near cap because of his LD effect that allows him to hit weakness damage. Imagine what Jecht BT+3 can achieve. All of the debuffs on the enemy are done via calls (Jack/Vanille/Auron).
Made a mistake of pushing the boss inside burst phase below 30% and it sucked Jecht's BRV, resulting in the next ability not being the plus version.
In this case, without BT effects, Garland is the debuffer, Vayne is the team buffer with his LD. Jecht does what Jecht do with his weakness damage, and he can regularly rebreak to trigger Vayne's BT follow-up.
Other synergy that I can think of:
Y'shtola: tried with Jecht in D. Pandemonium, works very well with her LD and BT effect and helping Jecht with his plus variant requirementand brv refund.
Ace: tried in D. Shiva, double trap card buff massively reduces defence and BT effect boosts a lot of teammates' damage potential
CoD and Vayne: tried in D. Shiva and D. Pandemonium and Aranea LC, the fact that Jecht can rebreak regularly means you can trigger Vayne in addition to CoD at the same time and push past phases with ease. Like in D. Shiva, you want to skip from 50% past 30% just so you can save your turns from one set of Diamond Dust.
3