Help needed to solve this one!

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?

所有跟帖： 

• 一點淺見 -October12- ♀ (184 bytes) () 06/25/2016 postreply 12:04:59

• Can it be extended from current size 7 triangle/28 cells? -gw88- ♂ (296 bytes) () 06/25/2016 postreply 23:52:04

• My thoughts -endofsuburbia- ♂ (403 bytes) () 06/26/2016 postreply 22:48:34

• 一點愚見 -jinjing- ♀ (240 bytes) () 06/29/2016 postreply 10:09:39

• 應該用矩陣來表示你的圖形。如下： -宇之道- ♀ (3041 bytes) () 07/01/2016 postreply 19:53:58

• Programming required to solve a triangle of larger size? -gw88- ♂ (590 bytes) () 07/01/2016 postreply 23:00:52