As others have said, a ‘panel’ would usually be 6ft or some are 1.8m.
3m would be a Closeboard (featheredge board) ‘bay’ – the post to post centre dimension – which would be constructed in-situ. and the last bay would be conztructed to suit the remaining gap.
If you are planning to use panels, the last panel can be made to measure or altered on site to suit.
Fencing basically is a modular system comprising of posts dug in @ 1.8metre centres, rails @ 3.6metre long and as many boards as you want clad ( typically 8 boards a metre at a hammer head gap) So if you want to construct this type of fence use these values to build the fence according to your required linear length. Obviously if a gate is required take this into account and increase the number of posts required. If a panel fence then they are still constructed in imperial so use this value in your equation.
Gate construction to follow in the next thread!!!
Depends on how big the slope. If using panels you can either step them or put in ‘gravel boards’ (usually 22mm x 150mm timber boards) under some of panels as necessary. If building a closeboard fence you can run it to the slope (or step/gravel boards as per panels).