I'm have 12 days in SE Asia. Do I have time to visit 3 counties? If so, which 3?

I fly into Bangkok and am interested in seeing Thailand and some other surrounding countries. I ume I have enough time to go to a neighboring country (Laos?) but am wondering if I should try to squeeze in a third. What do you think? What are your top 2-3 suggestions on countries to see, considering Thailand is already on the list...