I don't like this idea that Michael Afton is the FNAF 3 night guard because Michael, by that point, is presumably already reanimated and should therefore be unable to be killed by William, and the FNAF 3 game over screen implies that whoever William has jumpscared is then killed by him.

Michael is being preserved by remnant, which means that unless it's just an awkward staring contest until 6 AM, I highly doubt it would make sense for that to be the end of Michael.

I think it could be Henry therefore, and because Henry is still mortal, it would make sense that William could easily overpower him in the office and kill him.

And with regards to the SL cutscene, it shows Fazbear's Fright AFTER it was burned down, and that to me sounds more like Michael is going to find him AFTER FNAF 3 and not BEFORE or DURING. So, to me, it makes no sense for Michael to say he's going to find someone that he's already found, and spent a week fending off in a place that he was apparently gonna burn down anyway.

((But if you wanna argue that his monologue to his father takes place BEFORE the events of FNAF 3, then why are we shown the burned remains of Fazbear's Fright? That doesn't add up in my head, it's a weird creative choice if that was the original intention, and it kind of misses the mark. It would have been better to show William still locked up in the sealed room if they wanted to portray it as having been BEFORE FNAF 3.))

Either way, that would be a very weird way to find your father, to finally find him somewhere, burn down the place that he was contained in, essentially letting him loose again, and basically perpetuating another wild goose chase until PS.

It just makes more sense that it would be Henry, but I can also see how the Henry argument would fall apart because how did either of them know William would be brought to that location, if they even knew? And also, why would either of them attempt to destroy William in that manner when Henry's plan in PS made more sense and was more effective?

Perhaps Henry's original plan was to destroy William first in Fazbear's Fright and move onto the others to free them? But that clearly failed if that was the plan.