The BX3 is definitely smaller type of bag, where you still have plenty of storage space for discs as well as sidepockets for towels and other goods you carry around on the course.
Sidepockets are adjustable and its just enough to squeeze in a raincoat or smaller hoodie if the weather changes. Behind the sidepockets you have two bottle pockets (one each side) which can hold nicely a 1.5L bottle.
The top side pockets are not super accessible and actually pretty tight to slide in your phone or minimarker. However, once you stick your stuff there, there is little to none chance of dropping items out.
So called putter pouch at the top holds nicely 3-4 discs (putters). Personally I carry 16 discs in main storage space and 4 at the top.
I used to have larger backpack from Grip, and carried more discs, but BX3 made to review the disc selection and I actually prefer it that way. Made my rounds lighter for the back. Also the straps are very good- wide, soft from the inside but moderatly rigid from the sides to keep it nice and solid on your shoulders.