"三方晶系的" can be translated into English as "trigonal system (in crystallography)" or simply "trigonal" when the context clearly refers to crystallography.
In crystallography, the trigonal system is one of the seven crystal systems, characterized by a threefold rotation axis. If you need to be more specific in a scientific or technical context, you might say "belonging to the trigonal crystal system" or use the full term "trigonal crystal system."
For general use, "trigonal" would often suffice when the topic is understood to be about crystal systems.