They all grew out of the Indochina Communist Party during WW2. The party was originally mostly Vietnamese, but Lao and Cambodian membership grew as time went on. Both the Lao and Cambodian movements were supported and financed by the North Vietnamese, later just Vietnam since the Communists won their war. It was a close relationship between them, at least until the Khmer went batshit and started killing everyone. At that point, relationships soured some and the support from Vietnam lessened but by then, China had started providing support for all three countries.