Illumination. Artificial lighting was expensive in the 18th century, and one of an architect's principal constraints was getting daylight into as much of a house as possible. Glass fanlights above doors, to illuminate otherwise dark corridors, are the best-known example. Servants' quarters or work areas would be expected to manage with a mere glimmer of daylight. I've never seen this 3-holes version, but I've seen shallow lens-shaped cut-outs in stair risers in a couple of different places, and worked out that was their purpose - one was in stone in an old Highland castle, maybe Craigievar, and one in a Queen Anne mansion near Edinburgh.