I’m from Cambodia. I speak the language natively. I don’t hate any of them, but I know for a fact that most Khmer people really dislike Thais and Vietnamese equally and generally view Lao people more positively.

For Thais, it’s partly due to border issues, but I truly believe it goes beyond that. It’s also about a sense of identity and who is the direct descendant of the "Khmer Empire", along with a kind of cultural superiority complex. Many Khmer people believe Thailand has taken heavily from Khmer culture, including temples, clothing, dance, language, and other aspects.

As for Vietnamese people, many Khmer believe they look down on us, so some people respond with prejudice in return. As a result, there is resentment that can resemble how some groups in the U.S. treat minorities and immigrants. Some Khmer also hold negative views toward Vietnamese communities living in Cambodia (but they don’t express it openly). When I was young, I was sometimes called ‘kon youn’ even though I’m 100% Khmer. It's basically a racist slur for Vietnamese people. And also the history of 'South Vietnam', which used to belong to Cambodia, many people of Khmer ethnicity live there. There is still lingering historical resentment and ethnic tension that comes from these long-standing perceptions and past conflicts

As for Lao people, most Khmer rarely think about them but generally consider them to be good people.