Hiya, welcome back to the forums!
That's a tough choice, as all three are rewarding countries. Out of the three, I would be tempted to pick Cambodia. The main reason for this is because you only have a week, and it's a compact, accessible country which you can see a lot of in a week. In contrast, Laos, whilst wonderful, is harder to travel around (in my opinion), requiring a lot of overlanding. In comparison, you can fly into the heart of Cambodia with Air Asia or one of the other carriers, and enjoy Phnom Penh, Siem Reap and maybe Sihanoukville or Kampot all in a week without too much rushing about.
Vietnam is also a wonderful country, but I would suggest you would need a lot longer than a week to explore this beautiful place due to its size and how spread out the sights are.
Just my opinion though!
Enjoy your trip and do pop into the Thailand forums for recommendations.