Third degree tear is often an indication for CS. I don't think you'll have a problem getting one.
However, would you think about some sort of compromise? I only ask because first labours are often very hard. Long, grim labours ending in instruments happen relatively frequently the first time a woman gives birth. However, they rarely happen the second time. We frequently have women delivering in the car park/lift etc as their first labour was so long they assumed they would have plenty of time to get in! Pretty much every day at work I deliver a second time mum who SHINES with amazement and happiness that her second labour was so quick and easy compared to her first. And of course you have the benefits of a quicker recovery, less risk to you etc.
One of our consultants compares first births to an investment. They are hard and long. The payback, the benefit of going through a hard long labour is that the second one is quick and easy. By opting for a section, she says, you lose the benefit and payback of that first experience.
An option could be a labour with a low threshold for intervention... So if you are having a long latent phase, you abandon the attempt and have a section. Epidural on request as soon as possible. Short pushing stage, say 30 minutes and if baby not delivered or delivering imminently then section. That sort of thing. Truly, it's likely you will have a quick and easy delivery this time, so maybe preserve that chance with a detailed birth plan written with consultant and the supervisor of midwives that you revert to section as soon it looks like not being quick and easy?
Just something to think about. Best of luck with it all and I hope you get the delivery you want x