Hi, 15 days are ideal to cover Cambodia and Vietnam. Adding Philippines to it will make the trip hectic. Fly into Phnom Penh Airport (Cambodia) and visit Angkor Wat, Siem Reap, Phnom Penh in three days. On the fourth day take a flight to Hanoi, Vietnam. Spend a couple of days exploring Hanoi before heading to Halong Bay. Explore the surreal beauty of Halong Bay for a couple of days and then head back to Hanoi to catch your return flight. Fly from Hanoi to Ho Chi Minh City. Spend the rest of your days exploring southern Vietnam. For more information regarding both, check Tripoto's Travel Guide pages: https://www.tripoto.com/travel-guide/vietnam and https://www.tripoto.com/travel-guide/cambodia.
Hey, There is a lot of natural and historic beauty to be seen in all of these South East Asian countries, but given your time span, it is recommended that you skip Philippines this time. Here are some blogs and itineraries to help you out: