Some of the advantages of the 3 perforation system are:

It saves 25% of camera footage.

It is more suitable for HDTV and cinema wide screen (1.85:1) formats *.

It exposes more of the 35mm frame, resulting in better resolution **.

* When your camera is modified for SUPER 35 the aspect ratio
of 3 perf is almost exactly 1.78:1 (HD TV) and very close to 1.85:1
(cinema wide screen.)

** Using the 3 perf system and a SUPER 35 combination
you expose the largest negative area possible for wide screen
projection, with negligible cropping or waste.
AND, you still get the 25% film saving.