Is it better to fly or take buses?

Take buses for distances under 6 hours to see the countryside. Fly for anything over 8 hours to save your sanity and minimize fatigue.

How do I handle money across three countries?

Use a Charles Schwab or similar debit card that refunds ATM fees. ATMs are plentiful, but Vietnam's machines have lower withdrawal limits.

Planning a Three-Month Southeast Asia Route: Thailand, Cambodia, and Vietnam

Spend 6 weeks in Thailand, 2 weeks in Cambodia, and 4 weeks in Vietnam to balance transit time and exploration. Use the 'loop' method, starting in Bangkok and ending in Hanoi, utilizing a mix of budget flights and overland buses to maximize your time.

Secure your visas early. Apply for your Vietnam e-visa at least 3 weeks before departure, as it takes up to 5 business days to process. Check if you need a pre-arranged visa for Cambodia; most nationalities can get an e-visa or visa-on-arrival, but have $30 USD in crisp bills ready.
Establish your North-South corridor. Start in Bangkok. Head north to Chiang Mai, cross into Laos or fly directly to Siem Reap, Cambodia. Spend two weeks moving through Cambodia before crossing into Vietnam via the Moc Bai land border or a flight into Ho Chi Minh City.
Book long-distance travel in bursts. Don't book all your transport at once. Book your internal flights (AirAsia or VietJet) about 3 weeks out to keep costs under $60 per flight. Use 12Go.asia to book bus and train tickets 48 hours in advance.
Pack for dual climates. You will face intense tropical heat in the south and cooler, humid temperatures in Northern Vietnam. Pack breathable synthetic layers and one lightweight fleece or windbreaker for the mountains of Sapa or Northern Thailand.

Is it better to fly or take buses?: Take buses for distances under 6 hours to see the countryside. Fly for anything over 8 hours to save your sanity and minimize fatigue.
How do I handle money across three countries?: Use a Charles Schwab or similar debit card that refunds ATM fees. ATMs are plentiful, but Vietnam's machines have lower withdrawal limits.