Those three curses are unforgivable, at least in part, because there is no use for them other than to cause pain, enforce compliance, or kill.
Every other spell and potion mentioned has at least one use which is beneficial and/or benevolent.
For example, the Draught of Living Death mentioned in the comments is used to put the drinker into a deep magical sleep.
Assuming there is a readily available antidote, this would be an AMAZING tool for people in the medical profession - a potent anesthetic with limited side effects. A deep magical sleep would also help keep people from moving about and aggravating injuries, subdue prisoners safely, and give some relief to people who suffer chronic nightmares.
I'm certain there are similar uses for every other 'questionable' magic. And when such magic is used to ill purposes the law punishes them. The 'Unforgivable' curses are simply 'unforgivable', not 'the only uses of magic punished by law'.