If I am A, I will not rekindle it again. It's like NS, u do it only once.

after being together with B for 3 years, and had agreed mutually that there's no love anymore, I would consider the chapter closed because there's nothing to be achieve, what's to be done, had been done, the fireworks, the sunsets, etc.. It had ended. No more leftovers to pick up.
Having second thoughts probably because of familarity?
Why not? As B knows you best.
Further to that, if I have feelings for C than B, it further confirms that B is not for me anymore.
Anyway, it all about choices. The mind is a powerful thing, it can make you love B back and hopefully she feel the same way too.