Sort of. You're usually inputting C at a specific timing to cover both A and B used at the same time, but it can still lose to an individual option at a different timing or an entirely new one all together. In Persona I'll specifically look to cover throw tech and jump out being used at the same time for instance, but that leaves them room to throw or jump at earlier or different timings. It also leaves room for me to get mashed as well. A really good call out to an OS is as safe and rewarding as possible but looking for the specific reward on calling out the OS leaves room for different options to start working again. A pretty typical scenario I run into as Aigis is I try to cover fuzzy roll OS(A = block B = roll), but the main option I use to do that(option C) makes my mix up weaker and less rewarding (makes A stronger) but ensures I get higher reward versus the roll ( makes option B weaker). By calling out the roll OS I ensure I get high reward off calling it out, but I minimize my reward vs them just blocking without inputting the OS. If they try to mash roll without using OS timing I can still cover it probably, but my reward goes down as well. Additionally the way I do this leaves me pretty open to being DPed, where my normal mix up is DP safe.
I'm covering both A and B but not as well to focus on maximizing my reward for calling out A+B combined at OS timing. I'm also opening myself up to D(DP) which is a hard call out to my C. What's cool is if I understand they aren't doing A+B and are going back to just A or B, my options vs those open up significantly and we have actually come full circle to them using A/B/D and me hard countering them rather than looking for answers to OS.
It's really cool when something comes full circle like that. It's a hell of a rabbit hole.
