If you are a USA citizen, you could enter Thailand, visa free… And stay up to 30 days. At some point before those 30 days are up, go to Vietnam… There you will need to be preapproved for a visa, which is a simple process… And then go to Cambodia..... and get VISA on arrival.
After that, you can go back to Thailand and get another 30 days Visa exempt
if you have never been to Southeast Asia before… I would not worry about this… Because you want to spend more time in the neighboring countries. Laos, Malaysia,Cambodia… And especially Vietnam are such great places to visit.
flights are quick and cheap .
My recent visits to Thailand have been two or three months… So I make the boarding visits part of my plan.