Thailand’s currency is the baht, and at the time of writing the exchange rate for ease of calculations is approximately 35 baht to the US dollar. I didn’t take any baht with me to Thailand as I had pre-booked and paid for transport from the airport and our hotel, Trang hotel, takes card payment for meals. The local street vendors seem to take dollars for everything and the ubiquitous 7-eleven stores will take dollars and give change in baht. I did take plenty of US dollars in cash and was able to change it at one of the many currency exchange booths and banks in Bangkok, there are also plenty of ATMs there. Bahts are useful when paying for entrance fees to temples and boat trips.
The local currency in Cambodia is the riel, but here dollars are accepted everywhere and there is no need to change currency or get riel from an ATM. Change may be given in dollars with smaller amounts in riel. Most restaurants and shops do not take card payments. At the time of writing the approximate exchange rate was 4000 riel to one dollar.
In Vietnam it is easy to be a millionaire in Vietnamese dong as there are over 24,000 dong to the dollar. Fortunately most notes are in multiples of 1000 and even a 50,000 dong note is only worth just over 2 dollars. Some restaurants and bars leave the zeros off their prices so that 50,000 dong would be 50. Here again the dollar is widely accepted and change may be given in dollars and dong, there are plenty of currency booths and ATMs available. Card payments are more widely accepted here than in Cambodia.