Sorry but you wont be able to write jee advance as you can write it only for 2 consecutive times and this is your third time.

Yes. You can give an attempt in 2019 but it will be your last.

Thrice, once with your 12th boards, and two more times in next 2 consecutive years.

For example, consider a random student, if he gives his 12th boards (first time) in 2016, then he can appear in jee mains 2016, 2017 and 2018.Now if he has not given mains in 2017, still he can appear in 2018 (but not appearing in 2017 will not save a chance).

But if someone fails to fulfill minimum board % rule once and sitting again for improvement then he/she has only two chances left.