I was wondering the same thing, 3 days vs. 72 hours. With our projected landing time around 3:30 this Thursday, we wondered if we needed to wait until 3:30 Monday to submit the form. We went ahead and submitted it yesterday morning at 9:00, which is outside 72 hours but still within 3 days, figuring if it was too early they would notify us. Not a problem - my husband's approval came back around 10 am, and mine came through at 2 pm. So it looks like 3 calendar days should be good to go.
I think it's three calendar days (not 72 hours) prior to your departure flight to SXM. But if it's too early, no worries, it will tell you it's too early and you just re-submit when the time is correct.
You first must answer the question, are you fully vaccinated as per the EHAS travel requirements. If you are fully vaccinated than you have 3 calendar days, prior to the date of departure on your final leg, in order to submit your EHAS application. For example, If the final leg of your flight to St Maarten departs on Saturday then you can submit your EHAS application starting anytime on Wednesday or later. 72 hours no longer has anything to do with submitting your EHAS application. If you require a test It is either 48 hours(PCR) or 24 hours(Antigen) depending on the type of test submitted.