Well, in my opinion the best way to capitalize three weeks out is to ride as much as possible, sleep as much as possible, and eat really clean. At this point, not much can be gained in you fitness (cardio and strength) so concentrate on recovery and mobility (or flexibility). As far as arm pump is concerned, make sure you warm up really, REALLY well before your motos. This article by Coach Seiji is a great guide (
Race Week and Warm ups). Not sure about the magic powers of Beet root but what they heck, it's really good for you even if it doesn't effect your arm pump.