Help needed to solve this one!

來源: 於 2016-06-23 21:29:26 [舊帖] [給我悄悄話] 本文已被閱讀：次

The answer for below is 3 but which 3 cells should be marked with dots?  One may use #1-28 (1: top; 2-3:2nd layer; ...) to indicate these cells.

Besides trial and error, any good reasoning?