Removing Drop-in Sink Mounting Clips
I would say remove the triangle braces, an oscillating saw would work well (this is the one i own) or break them out with Channel lock (adjustable) pliers. I suppose you could drill a hole through them as well so you can get a screwdriver on them.
You can glue them back in after the new sink is in.
For the middle one you may be able to get a screwdriver in behind it and manipulate it out enough to get another screwdriver on it. Or cut away a notch to access the head.