This is when you have a hand with three tiles each of the green dragon, red dragon and white suits, which are extremely difficult to make up into full sets of four. If you do manage to make them up it's a "triple grand flush," which is used to describe the height of satisfaction.
表示牌中有青發、紅中、白板三種牌各三張，但是非常不容易湊齊，若能湊齊就叫「大三元」，因此被引申用來形容圓滿的境界。