The "3 days" seems to confuse people. 3 is 3. It's not 4 it's not 6 or 10. It's 3. Just count backwards from day of arrival. Apply on any of those 3 days.
But it's easier with a visa. People can get a visa before arriving then open a bank account. It's not easy for tourist in UK or USA to open a bank account, Thailand's no different.
or you apply in your own country for the visa. No Thai bank needed. Then extend in Thailand after opening Thai bank account with your visa entry and depositing 800,000. (That's your nest egg) I'd guess having that is like a guarantee you've enough back up money.
your questions been answered by several people. There's no new visa requirements. There's and entry card done on-line up to 3 days before you arrive back in the country. That's any time during those 72 hours prior to your return.