From an historical perspective, your question begs another question.
What is a three-year B.A.?
Historically, isn't the 4-yr. B.A. rooted in the concept of Grade 13
plus 3 years?
Under a former system, in 1965 I graduated from the U. of A. with a
three-year B.A. after Grade 12. At that time, B.C. had Grade 13 plus
three years. Today B.C. has Grade 12 plus 4 years for a B.A.
Being just as provincially narrow as you are, I'm interested in
how many years Ontario's Grade 12 students need to get a B.A.
What's the situation in other provinces--anyone?