hi big bear, hope your well in, having a third degree tear will naturally make you anxious about this birth, have you discussed using water to reduce the risk of tears, sitting in a bath and massaging the perineum(the bit that gets torn) helps the elasticity and flexibiltiy of the skin, using oil is also recommended. sitting in a bath as soon as you start to get those niggles of labour could be starting is a real good way of relaxing the whole area, the more relaxed you are the less likely you will also tear, havign a water borth reduces the risk, and also certain positions of labour, chat about all of them with your midwife. i had 2 babies in 2 years both water births and no cuts no tears despite my second being the wrong way round and getting stuck at my tail bone junction and having a horrendous time at one point talking about emercency c section as he was stuck and not moving down for ages! having a realy good midwife makes all the difference.
good luck xxx