I'd be getting a second opinion from a midwife. Most women who have had a third degree tear do not tear again, third degree. I do not agree with the advice of your physio, nor do I believe a physio is qualified to give such advice.

An OB will generally recommend surgery when there's the slightest indication for surgery as this is their specialty. So I'd recommend consulting with a midwife for a second opinion, to get some balance to the info that you're getting.
I know this is an old thread, but I couldn't go past this. How is a physio not qualified to give advice about the pelvic floor? And why is a midwife the most appropriate person to give advice to a woman with a 3rd degree tear who has had fecal incontinence and urgency. These in themselves complicate the issue and increase the chances of further nerve damage occurring. This is not in the scope of practice of a midwife. Yes if the woman chose to birth vaginally, then we could provide advice about ways to minimise tearing. But management and advice of an anal sphincter defect that is symptomatic is the remit of an obstetrician/general surgeon and a physio (prererably ones who have specialist training or interest in the area.)