You will need at least 5 days to see Cambodia. The reason behind this is that The country is huge and there is a lot of points you could visit. You will have to fly to Phnom Penh first, then Ho Chi Minh City and then Siem Reap which is the most important tourist destinations. If you want to visit the south of the country then you will have to go via Ho Chi Minh City. I would suggest you to plan your trip in a way that you visit Siem Reap for 2 days then go to Phnom Penh for another 2-3 days and then back to Siem Reap again. That way you will get to see the main highlights in the country.