Celebrating 64 years, I'm eager to explore Thailand, Vietnam, and Cambodia for 90 days Maximum. Please advise on the visa requirements.
TLDR : Answer Summary
For a 90-day trip to Thailand, Vietnam, and Cambodia, you can enter Thailand visa-free for up to 30 days if you are a USA citizen. Before your 30 days are up in Thailand, you should travel to Vietnam where you will need to obtain a preapproved visa. In Cambodia, you can get a visa on arrival. After visiting these countries, you can return to Thailand for an additional 30 days visa exemption.