I think you might benefit from a second opinion.
I've heard that if you haven't got significant problems with incontinence then a vaginal birth after a third degree tear is unlikely to result in further perineal problems.
However, if you already have problems with continence then it may be worth considering an operative birth.
On my part - I have minor problems with my pelvic floor. I leak urine when I run or sneeze and have mild fecal incontinence in the sense that if I have to go - I REALLY need to be near a loo(sorry TMI!). I always think it can't be that bad though as I've never been caught out in public!
My pelvic floor problems are related to a forceps birth with my first 9lbs 6oz baby, and a shoulder dystocia with my 10lbs 12oz second baby.
Despite all this I never, ever would have wanted to have an operative birth with my third - but that is just me, as I would hate to have a c-section, really hate it.
I tried to prepare myself for my third birth by eating a diet very very rich in fruit vegetables and omega 3 oils. I took a supplement (mum omega) and ate my two portions of oily fish a week, plus nuts and seeds. I think that when it came to the birth I was very 'stretchy' - plus I worked on relaxing my perineal floor so I could just let my baby out. My third birth was great - I only had a tiny graze.
I also had a very good midwife.
If I was in your shoes going for a vaginal birth I'd want to meet with a senior midwife first to talk through how second stage was going to be managed - birth positions, breathing, hands on or hands poised (in the sense of whether the midwife would support your perineum during the birth) and try to work on my visualisation and breathing skills for second stage and crowning.
I do wonder how many follow on tears are linked to mothers being too fearful and tense to let go of their pelvic floor, relax their perineum and just breath the baby out , or a surge of adrenaline at the last minute (again linked with fear) causing uncontrolled mad pushing in late second stage.......