How do I open this flush ceiling light to change the bulb?
The installation instructions seem pretty clear.  You should remove the three screws around the perimeter and then you may have to twist the shade portion to pull it down.  The bump visible next to the screw could be an indentation that holds on to a notch in the shade that requires it to be twisted before it comes off.

Maybe you just need to use a little more force than you did, but that diagram seems to show that there's nothing else holding the shade on.