While analyzing screenshots of the movie, I noticed something interesting relating to the triplets (the more polite name for them, as their official name is the Bimbettes). For ease of reading, I'll separate the things I noticed into three galleries:
During the "failed wedding," as you can see above, the triplets were initially content with setting up the wedding (or at least Paula was), but then their smiles were turned upside-down so to speak when they noticed Gaston was there, as especially evident with Paula and Claudia's reactions (note, Paula's the one with the green dress, Claudia's the woman in red, and Laura's the sister in amber). Laura whispering to Claudia and possibly Paula may have something to do with Gaston's arrival. This seemed to imply that although they were aware that Belle was likely getting married, they weren't even aware of who the groom for Belle would be until Gaston stepped out, thus meaning they were kept in the dark about Gaston's involvement.
During the beginning of the song "Gaston," as LeFou is singing "And it's not very hard to see why!", the triplets were noticeably hesitating for a good few seconds before giving loving comfort to Gaston to make him feel better. I'm not sure what that noticeable pause was for, whether it was politeness to LeFou to let him finish, or residual anger or grief for Gaston basically keeping them in the dark like that earlier, but whatever it is, apparently something happened that resulted in them, especially for fangirls of him, hesitating.
Finally, during the reprise, if you notice from image files 3423-3436, they were looking at Gaston and even bending over when he and LeFou are signing. However, when they make the roundabout to return to their position (starting at 3442 and ending at 3445), the Bimbettes aren't looking at Gaston at all. Image file 3442 started around the point where LeFou sings "persecutes harmless crackpots like Gaston!".
In addition, while the chorus is singing their final verses, if you look closely, the triplets aren't singing along (the fact that their mouths are still stuck in that almost Stepford Smile position while the other villagers are clearly moving their mouths to the lyrics are a dead giveaway that they aren't singing along, or if they are, they're doing so through their teeth). This and the other part above relating to the reprise seems to imply that they may not have supported Gaston's plan, or rather, what little they knew of it even among the rest of the villagers: The triplets were absent when Maurice burst into the bar and ended up thrown out shortly thereafter, and going by a pan-away shot beforehand at the end of the song, with the possible exception of Paula (and even she was clearly on her way to the kitchen to prepare orders by that point), they most likely weren't even in the room when Maurice burst in, ended up thrown out, and Gaston loudly declared he came up with an idea to get Belle to marry him. In fact, they don't even reappear until right when Gaston and LeFou, in reference to the plan, said "Let's go!". In other words, they most likely didn't even know that Maurice burst into the tavern, much less that Gaston planned on doing something to Maurice to force Belle into marrying them.
Anyways, just some details I noticed. The last bits aren't confirmed, and for all we know could be animation errors (or at least the bit about their not looking at Gaston).
Edit: For some reason, image 3451 is not showing up.
Edit 2: Okay, 3451 was never actually uploaded (copycat image), and correction, it's four galleries, not three.
Well, if my analysis on this is correct, I guess we have the answer as to why they weren't among the mob at Maurice's house: Not only do they not seem to support Gaston's stated actions, they apparently may not have even been aware that Maurice arrived at the tavern or even be aware that Gaston came up with a plan.
Noticed something else, Paula in particular actually seemed to be fearful (specifically, she seemed to grip onto Laura's arm while looking horrified at Gaston). I wonder if there was more to their sobbing than just the fact that Gaston's marrying Belle over them (After all, I don't think "fear" would register to people who are simply passed over regarding courtship, not unless they literally depended on it with their life, such as the mob breaking their legs, literally, if the person they tried to woo went with someone else). I mean, their reactions indicated fear, apparently.
They're the Bimbettes. And yeah Gaston and LeFou probably didn't tell them the plan because why would they support Gaston marrying another girl?
Noticed you're talking OOC (Out of Character), since you referred to LeFou as if he were another individual.
I know that's their official name, but I'm more of the idea that if they were truly stupid, they probably would have reacted to Gaston marrying Belle in a similar manner to how Ricky Ray Rector handled his last meal before his execution (in case you're unfamiliar with what I mean by that, Ricky Ray Rector was a man who ended up having a failed suicide when about to be arrested, resulting in brain damage. He suffered extensive enough brain damage that, when about to face execution and ended up having his last meal, he ended up neglecting to eat his last meal, saying that he'll "save it for later." It was because of this that they actually tried to repeal his execution because he was mentally deficient, which violated the ban of cruel and unusual punishments in the Constitution. Ultimately it went through.). In other words, they'll think they can still marry him because they cannot comprehend how, if Gaston married Belle, they cannot have any shot at Gaston.
And by plan, I presume you mean the failed wedding. It's clear that they weren't aware of the blackmail as they weren't even in the room when Maurice burst in and was thrown out (well, Paula might have been, but she was already on her way to the back room so she most likely just missed it). As far as whether they'd support it, they seemed supportive enough to be shocked and angered that Belle actually refused Gaston's advances, something they would not have done if they weren't supportive at all.