Is it too late to do the TDAC before flying to Thailand?

No it’s upto 3 days before or can do in designated booths on arrival but best to do before to save the delay when you arrive

It's within 3 days. Not 3 days before.

You can do it in the line to check in.

You can only do it in the three days before you arrive in Thailand.

You can do it 3 days before, or 2 days, or 1 day.

You can do it on your flight if they have wifi.

You can do it when you arrive at the airport on their WiFi.

Or you can do it at a kiosks before immigration.

You can do anytime after 3 days, you can even do it at the airport when you land in Thailand, don't worry

You can do it anytime before arrival or at kiosks when you land.