If you have an appointment the third Monday of the month -- then you count Mondays. If you have an appointment the third week of the month and the month started early in the week, then you count as if the month started on Sunday. If the week started late in a week -- ask.
English is confusing when it comes to time and dating. Different countries and regions handle it differently.To me this Monday is Feb 27th and next is March 6th. To my husband, next Monday is Feb 27th.
The most simple fix is to be specific. I will see you on the 14th of March.
On edit: If you were running a scientific experiment or a report and the time frame was three weeks, count 21 days from the time you started. If it isn't specific, still count 21 days and check to see if things are coming to fruition.