b arts psych is really an arts course, with a psychology major, and an honours year built into the fourth year. it's basically streamlined to get you into the honours. you need a distinction average in second and third years. administered by the arts faculty.
b psych arts is a psychology degree, which essentially does the same, but you need a credit average in first year, distinction average in second and third years to get into the honours course. you can get a second arts major. is administered by the science faculty as an arts stream of the b psych degree.
b psych science is a psychology degree, also steamlining you to get into the honours year. you need a credit average first year, distinction average in second and third years. you must do 12cp of maths in first year (as morbo has stated) and your second major must be a science one. adminstered by science faculty.