Who are the three old and cold friends?

Three Friends in Cold Age: Traditional Symbolic Patterns of Han Nationality. It means that pine, bamboo and plum don't wither in winter, so they are called "three friends of the old cold". Pine, bamboo, plum proud wind, standing in the frost, the spirit is commendable! Pine symbolizing evergreen, bamboo symbolizing gentleman's way, and plum symbolizing purity and beauty, because it can still be evergreen in cold winter.

Source: Song Jingxi's "Wang Shou Ji": "That is, living in a tired place, growing hundreds of plums, and being a cold friend."