I think you have enough time to travel 3 of these countries if you arrange your time perfectly.
Firstly you find out the cheap flight from your country to Siem Reap, you will stay in Siem Reap around 3 days 2 nights to visit Angkor Wat and use bus from Siem Reap to Phnom Penh. Then consider a fly from PHN or bus aslo to Saigon.
In Saigon to visit your friends for around 3 days then fly with jetstar or vietjet (cheap airlines in Vietnam) to Hanoi. In Hanoi, you can take your time to Halong Bay (should visit Halong Bay - one of world heritage site) and stay in Hanoi at least 2 nights.
And fly with air asia or jetstar to Thailand, sometimes they have very cheap flights :D
I have experienced to travel in budget! so reply to me if you want to have more information,