It seems to me that the real problem is not how common or uncommon a third degree tear is, but that so many of you have experienced really appalling maternity care. The original poster sounds really angry (understandably) about the care and information she received.
If a third degree tear occurs despite everyone's best efforts and is managed well afterwards it is not neccessarily particularly traumatic, from my experience
I had a third degree tear with my first baby, but it was pretty much inevitable (as opposed to caused by poor care. I had a forceps delivery, but it was either that or an emergency section, I think the forceps + third degree tear were better for me
I felt it was all managed well, I was stitched well in theatres by an experienced registrar, had good pain relief and a good recovery afterwards. No issues with incontinence. Follow up with consultant obstetrician and specialist physio. Offered c-section for second baby if I wanted, chose a vaginal birth which was fine.
I didn't know much about tears prior to giving birth, but I don't really think it would have been particularly helpful to have know more. I don't know anyone else who has had a third degree tear, but I can't say I feel marginalised and isolated and I'm perfectly happy discussing my perineum with random strangers. I think this is because it was all managed well, and was the best birth possible in the circumstances.
ebd, would birth trauma counselling perhaps be useful? Have you complained to the hospital where you gave birth?