What research suggests that people grasp information better when arranged in groups of three?

The author didn’t cite any source for this "Research into communications theory".

      Why should you aim for two or three major points? Research into
communications theory suggests that people grasp information better
when it is arranged in groups of three. While you need not adhere
to this rule slavishly, a three-tiered argument gives the appearance
of balance and strength.7 In addition, it is unlikely that all seven or
eight points will really stand alone: no fact pattern is that complex.
It is much more likely that some of those points can be combined
in some way and those that cannot are either mere spectres or too
minor to merit a mention.

Page 17. Stacie Strong BA English literature (UC Davis 1986), MPW (USC 1990), JD (Duke 1994), PhD Law (Cambridge 2002), DPhil (Oxford 2003). How to Write Law Essays & Exams 5th Edition (2018). p 47.