I'm quite sure that Kiara herself has reinforced that if you see her performing on stage, it will always be her, and I'm inclined to believe her since she's probably to prideful about her dancing ability to have someone else stand in for her. As for Gura, she's way too emotional about her performance for me to believe it is faked. As for Ame, her performance in 3rd Fes mainly involves her doing the exact same motion for 70% of the song, kinda a waste if they hire a professional dancer just to do that.
People underestimated how complicated (and possibly awkward) it is to hire a stand-in, they would have to ask them to be synced with the singing and also make sure their movement is "in-character".