What are these three indentations on the SBD's wing?
According to "The Dauntless Dive Bomber of World War Two" by Barrett Tillman, they are "letter-box" anti-stall leading edge wing slots added to improve the planes stability when approaching the 78knot stall speed during landings (and take offs, actually.)

The initial versions tended to not be able to build up enough speed during the (catapult-less) take offs from the carriers of WW2, and would simply drop off the front edge of a carrier. Adding the three slots basically fixed (or lowered) the speed of stall.