Gillyan, I am vehemently pro natural birth and I too had a 3rd degree tear - having had an active labour I ended up having to stay on my arse and tore 20 mins later. I believe that my bad birth position plus pushing hard were key factors.
After the trauma of a long and arduous recovery I swore that I would rather have a C-section than risk another similar tear. However, 2 years on I have changed my mind. Now that I know the reasons for my tear and what I can do about it, I will take better control of the birth next time around - as suggested in the radmid website, I will stay on all fours no matter what, and make the medics work around me rather than place myself in a position that's convenient for them.
That said, I had a diagonal episiotomy and so the colostomy bag threat was nothing like yours. That is a real problem and so I understand your worry. Perhaps the thing to do would be to have an elective episiotomy and then labour gently on all fours, letting the baby slip out rather than pushing him/her out?
Can you recall what happened in your labour/ birth that might have led to your tear? It's important that you think this through to help you as your decision has to benefit you and you alone.
Also try to find specialists (such as a doula and a midwife) who have different perspectives. Get as much information from them as you can. Marslady is a MNer, a doula and a wonderful woman (who's had 5 babies of her own!), so I'm sure she'd be more than happy to advise you, and help you in the birthing room if you can afford it.
Best of luck.