I would leave them in for the 3 weeks as it will help compact the lees so they wont be fluffy and they wont be so easily stirred up when racking. Another tip is to tilt your carboy using a wedge so that when you go to rack off the lees, they will all be on one side. I do this 2 days before racking. If this is a kit where you have transferred the lees over from primary bucket then dont wait to much longer, if this is not the lees from primary then it is not as detrimental and could actually add to mouthfeel and complexity of the wine. Aging wine on the lees this way is called Surlees.