This sounds really optimistic. People said the same thing about season 2 with the tournament arc and the Stain arc, but there hasn't been a big boost in volume sales like people were hoping (besides backlogs). I want MHA to be a million per volume seller as well, but I doubt that will happen. Season 3, if it even happens, has to really draw people in for it to cause a large boost in sales. I'd assume that season 2 will end with the final exam arc, then if season 3 does come, they can fit the school trip and hideout raid arcs (28 total chapters) into a 12/13 episode season.
Also, looking at the story arcs on the wiki, I realize just how fast the pacing is in MHA (whether that's good or bad is debatable). The arcs are only about 10-20 chapters each.