You'll probably want to eliminate a country (or two) out of that mix. Also don't forget to do your visa research.

1. 3 nights
2. Have not been there
3. 2 nights if you're also going to Halong Bay, or 3 nights if you're not.
4. 2 nights. Afternoon flight arrival, sunset at Angkor, 1 day Angkor, 1 morning Angkor, evening flight out.

You have 3 weeks. It looks like you should have 10-14 days for Thailand, 4-5 days for Hanoi/Halong, and 3 days for Laos or Angkor. Busy, but not too busy. The flights will be busy between Chiang Mai, Bangkok, beach place and Hanoi. I don't see direct links from Chiang Mai or Phuket to Hanoi or Siem Reap so you'll have to back track to Bangkok.

Anyways, just my two cents. 3 week honeymoon. Nice!!!!