Birth control ring

Okay so I did do my research but im still confused. I'm due to start back on birth control soon so I want to be certain onthe type i want.

The whole skipping period thing i know it's 3wk, 4wk or 5wk then you have to take it out at the end of that week. Do you have to leave it out for 7 days to get your period or can you go straight for a new one and leave it in for another 5wk. Then replace it again right away for a new one for another 5wk etcetera.

why would I replace it every 3wk if it can last up to 5wk ??