Ahhh I feel you, 3rd degree tear memang scary thinking about next time! Honestly the best thing is to properly discuss this with your doctor or midwife before labour, they can assess your scar tissue and give you a proper plan (some do perineal massage exercises, some recommend different pushing techniques). A few of my mum friends who went through similar tears said doing the exercises their physio gave them made a difference, and one swears by having an epidural so she could push more controlled-like without panicking. Most importantly make sure your doctor knows your history so they can monitor closely during labour and help guide you through the pushing stage, don't hesitate to ask them all your worries, they've seen this before!