Is it too late to complete the TDAC before my flight to Thailand, and will I be denied entry if I don't?

No, just try & do it now.As it's so new there will be thousands of people that won't have done it.

They will have kiosk at BKK for those who failed to do it before landing.

the first month it can even be done at the airport in Thailand

I went through immigration at SUV this morning and some officials will ask to see your confirmation QR code. Some people had printed out the two page email, others had it on their phone. It was a miss. Seems the older staff just were paying attention to the passport and biometrics and the newer younger staff were more cautious. Though immigration was jammed pack those who were checking the forms told people to get out of line, line in the app and return into line.

It is 3 days OR LESS before entry, not 3 days or more.

I got asked by Qatar before they booked me in my flight. It was easy enough but thought it was interesting they wanted to see before proceeding.

Between 0 and 72 hours before landing and you pass through immigration.

Any time within 3 days, there are also tablets set up in arrivals so you can even do it there when you arrive.

✈️🇹🇭For example, if you are arrival to Thailand on May 10th, you can fill out Tdack on May 7th, 8th, 9th or 10th. 👍

It’s 3 days maximum. You can even do it in the queue for immigration once you’ve landed