Cephalantheres are generally light undergrowth orchids. The flowers open little. To distinguish the 3 species, it's quite simple: the red phalanthera has pink flowers, the long-leaved phalanthera has leaves which start from the base of the plant and form erect swords, the Large-flowered Cephalanthera has rounder leaves along the entire stem. 

You should not always rely on the colors of vernacular and even Latin names. The white Cephalanthera has more yellow flowers than the Cephalanthera longifolia and the red Cephalanthera ( rubra ) has pink flowers... As for the size, I found many Cephalanthera longifolia larger than the Cephalanthères blanches (Large white helleborine in English).

It is therefore essentially the shape of the leaves that we will decide between Cephalanthera damasonium  here presented and Cephalanthera longifolia .