Cambodia + Thailand
The Thailand–Cambodia route is one of the most popular in Southeast Asia. The Bangkok–Siem Reap bus takes about 8 hours via the Poipet border crossing (see our article on land borders). Important: the official Cambodia e-Visa site currently lists Poipet as temporarily closed to entry with an e-Visa, until further notice. That does not necessarily mean the crossing itself is closed (evisa.gov.kh). By air, Bangkok–Siem Reap takes 1 hour 15 minutes with Bangkok Airways or AirAsia — more comfortable for a short stay. A typical pace: 5 days in Bangkok and Chiang Mai, then 4 days in Siem Reap and 3 days in Phnom Penh.
Cambodia + Vietnam
The natural link is the Phnom Penh–Ho Chi Minh City axis (bus about 6 hours, Moc Bai crossing). For a 3-week trip: Siem Reap (4 days) → Phnom Penh (3 days) → Ho Chi Minh City (2 days) → Mekong Delta (2 days) → Hoi An (3 days) → Hanoi (3 days). The reverse direction works just as well.
The triangle route
Bangkok → Siem Reap → Phnom Penh → Ho Chi Minh City (or the reverse) is the most travelled multi-country route in Southeast Asia. Allow at least 14 days so you are not rushing. Regional low-cost flights (AirAsia, Vietjet, Cambodia Angkor Air) make quick repositioning possible without losing whole days to overland travel.
Visas and logistics
French nationals do not need a visa for Thailand (30 days) or Vietnam (45 days since 2023). For Cambodia, the tourist e-Visa (Visa T, 30 USD, evisa.gov.kh, 1-month stay) covers the entry points where it is accepted. Temporary restrictions on entry with an e-Visa can apply at some land borders. Check the official list before you travel. Tourist visa on arrival — available at international airports. Check the official fee in force before you leave. If you return to Thailand after Cambodia, your first 30-day permission may already be used up — check your original entry date.