I'm looking at a flight on Dec 24th.
Chiang Mai to BKK on Bangkok air arrives at 8:15 AM.

Flight to SGN on Vietnam air leaves at 11:20

Is 3 hours enough time?

Both flights use BKK? What airline are you fly from Chiang Mai on? As long as your flight is on time or even an hour or two late, you will be fine..

Probably not, but why not just use Vietjet who fly direct between the two cities?

Clive, you think three hours isn't enough for a domestic to international transfer at BKK? I've never had it take even an hour...But your idea of a non-stop from Chiang Mai is a good one. I didn't know about the Vietjet flight.

Sound advice always is to never risk a Domestic Flight linking up with an International Flight on the same day. Sh*t regularly happens, particularly with the Domestic Flight, weather, schedule changed at last minute, delays and even cancellations.

Vietjet are a very good low cost airline. Their tartan thunderbird uniforms are ab fab too.

I think it's plenty of time. I've found the entry into BKK from other countries can be very slow, but you won't be coming through those immigration lines. Bangkok Air flights tend to run on time, IME.

Hopefully, you have travel insurance, just in case, there is a major delay! The Vietjet is a good suggestion, as it will involve a lot less air time and the transfer. Might even save one baggage fee.

ODDLY, They don't have that flight on Dec 24th.....They have it on Dec 23rd and on Dec 25th.
Go figure.

Yes, that was my thinking.... If I were arriving in Thailand and then trying to leave, I'd not do it.

Bad weather can cause chaos to Schedules.....sustained bad weather even worse. That is reason enough not to try and have connecting flights on the same day.

Always stay overnight between flights, and completely remove all stress.

I would fly to Bangkok the day before. Better safe than sorry! Use the final night for last minute shopping. See the city for one final night. In the morning you can use the Airport Rail Link to go to the airport. The Rail Link connects with the Skytrain if you want to stay in another part of Bangkok not directly on the Rail Link route.
UrbanRail.Net > Asia > Thailand > Bangkok Metro

Or, take your chances!