I dont know why it took so long for them to send an email. Tracking shows our application got there on 7th March... grrr
If they're busy, it can take a few days for them to log all the applications. It could be that, say, 500 other applications arrived at the same time as yours and yours was the last one, and it took those 3 working days between March 7th and 12th to log them all into the system and trigger the emails.
Back in summer 2016, there was a massive backlog and it was taking 2 months for non-priority applications just to receive the 'documents arrived' email... and then another 2-3 months after that for the visa to be processed!