Just suggestion, You got great chance here to document, the best you can, how this works out for you. Meaning several things, what phase of training are you in at this time, what surface did you do this three weeks on? How are you feeling, legs, energy and motivation etc. All suggestions are good ones, but as being said, many things need to be locked into your memory as to what works for you.
If we are talking crash training or hard three weeks and you reduce mileage or run easy ...whatever you do, I would Document it down on paper or computer. How did you feel, what surface are you on, how quickly did your body seem to feel some recovery. If you have a race how did you feel and was it areobic problem or you just could not seem to lay down the speed but you felt great otherwise.
Just suggesting write it down REMEMBER, what was your nuition in these days of reduced mileage and remember everything you can....so in the future you may try a variation of these easy weeks your doing but be able to match things up with other factors from before and ZERO in better with what usually seems to work best for you. Sorry so long.