If my professors didn't respond to my emails about recommendation
My mother used to say (this was in the days before the internet) that the moment the letter she'd written got picked up by the mailman, she was ready for an answer to arrive.

Three days is too soon to worry.

Normally one should wait two weeks before starting to worry.  Try to wait two weeks.

If you really can't sleep, though, I suppose you could write again, briefly and politely, after one week.  You might also visit them in their office hours to ask in person.