Taking a close look at the screw that holds the bracket in place seems to show that you will not get that out with a conventional screwdriver.
This looks to me as if it is a hex headed machine screw. As such, removal will require another type of tool like a 6-point socket or a nut driver as shown below:
Some sockets are available that have a flexible joint that permits their use in areas where straight in access to the fastener is not possible:
Be aware that sockets and nut drivers come in a variety of sizes and so it would be essential to obtain and use the proper tool.
Note: This fixture looks like it would be totally possible to remove the glass dome / dish from the light simply by removing just one of the brackets. You should check more closely at all three brackets. It is possible that one bracket is held in place with a different type of screw that is easier to remove with either thumb power or a standard screw driver.