Obviously (?) its used for boring three holes at a time, or maybe slots if the drill mounting block is designed for sideways forces (i.e., not a tapered fitting without a drawbar). But why is this particular configuration so useful as to have a large stationary machine dedicated to it? What type of joinery or other fabrication makes use of this?
From the example photos below, you can see the 3 boring spindles are relatively close together and possibly all at the same height. Although it looks like the outer two may have adjustable positions.
(I think the triple-spindle portion may be a removable head, in which case this machine might serve other operations as well. I can more easily imagine what a single-spindle boring machine would be used for. But > 1 not as much.)