On your first point, I am not sure, I actually have friends in Europe who just visited Myanmar recently, except the "photography at the boarder" thing, they did not complain being restricted. I will ask them about their experience once again.Myanmar is country very closed, tourists in visit are photographied upon arrival is not a free country.
Two others are members of ASEAN and have good relations.
Thai T-84
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On your second point, I disagree. Firstly, all three are members of ASEAN. Secondly, Thailand and Vietnam were at odds at least up till the end of Vietnam's occupation of Cambodia in 1991. The weapons and materials to the anti-Vietnam factions from China and U.S. were passed through Thailand during the whole time. I don't know about the recent development between the two, but I am sure the word "good" is far from the truth. On the surface, governmental relationship may look like flip-flop, but underneath to make good relationship need lots of incentives or a common adversary which are not there to be seen.