Entry visa from Vietnam, then to Cambodia and then to Thailand.

You can get a Cambodian visa in advance in China, and it is definitely no problem to enter Cambodia from Vietnam. You may have to tell the Cambodian Embassy that when you apply for a visa, at most, you enter from a port in Cambodia, and you enter from Vietnam.

The second question is the order of these countries, right? Vietnam-Cambodia-Thailand or Thailand-Cambodia-Vietnam is ok, but China and Thailand are not bordered and can only fly directly from China to Thailand. I suggest you go to Viet Nam-Cambodia-Thailand, because Thailand can get a China passport to landing visa, but you can't leave the country without a visa (China border inspection won't let you go), but if you go to Thailand directly from abroad, there is no problem. So I went to Vietnam with a Vietnamese visa, then to Cambodia, and then to landing visa, Thailand. This can save a visa.

The visa scheme for flying is the same as that for land.