You can leave it in "primary" for the full three weeks without a problem.
Apparently around 15-20 years ago, there was an understanding that moving to "secondary" was a good thing - in that it prevented off flavors from autolysis. Currently, most people state that there is more risk from moving the beer to the secondary container than there is from leaving the beer in "primary".
As I'm becoming more of a "lazy" brewer, I'm not moving beer to secondary except for special circumstances. The beer doesn't seem to notice not being moved. .