A lot of options
there really are different options that you might explore...
If you are sure you want to do it at some specific time three weeks from now, you put it in the calendar (for example dentist: december 15th, 5pm)
If you are sure you want to do it around three weeks from now, you can put it in the tickler (for example: call dentist when he gets back from his holidays: sometime in december)
Occasionally I put this last type of items in waiting for (for example: "waiting for: dentist back from holiday in december to call him")
I know some people use their someday/maybe list very actively to park any action that they will not be doing in the next week or so. I don't have that habbit.
So you see, there are different options... I would suggest you just pick one, and try it. If you like it, keep on using it, if you don't, try something else...