Did a bit of a deep dive yesterday! the episode they are killed off in is episode 20, but they have appearances in many of the other episodes in the 10s,,, they assist the heroes in searching through a tomb in one of them etc. What I find interesting is that the three who die are semi integral to the plot and the main villain, yet after the clip shown they are never shown or even mentioned off hand. In a previous episode the main villain girl sacrifices herself to save one of her comrades yet when they are actually killed she doesn't even seem that bothered. Guess thats being a zako for you huh.
here is a Dailymotion link to episode 20 if you go through the channel which posted it you can find the rest of the episodes (the quality is a little bad though), also if you go to the clip originally shown there are a bunch of other scenes in the recommended I think this one does a good job of giving an overall run down though there is a funny filter over it
https://www.bilibili.com/video/BV1Qp4y1D7fo/?spm_id_from=333.788.videocard.2