Yes those BG teams could compete against 3A schools. Not today's BG teams, which is still a good program/team, but not world beaters anymore. But those State Title BG teams could have played a 3A schedule and competed. Still if you forced them to 6A it would have been a safety concern.
You are ignoring my arguments that Blairsville or Penn Manor playing against BG once a year is not the same as Canevin or even BG team that you say could have competed in 3A, playing a full schedule of 6A teams with a schedule looking like Butler, Hempfield, Norwin, BP, Mt Lebo, North Allegheny, Pine Richland, Pittsburgh Central Catholic. There is no way a school that size can complete a full season like that. Those schools are now playing and in some circumstances, starting 9th graders as there is no freshman or JV teams. All those 6A schools have freshman and JV teams and if you are 9th grade playing and starting varsity it is because you really are that good, not because you are a warm body like these small private schools.