Why was I attacked and plundered even with my 3 day shield up?

You should not be able to be attacked and raided with a shield up. Enemies can attack with missiles which can damage your base (temporarily) and possibly make you temporarily to a new location but they cannot raid your resources. Some missiles can destroy some resources / kill troops. check your mails for battle reports or missile attack reports.

If your battle reports show that you were raided by an enemy player while shielded then this is a game bug and you should report it to the game support / developers they may compensate you for resources lost.