Three weeks isn't very long so no big worry about the fuel in the tank. Just don't brim it because not only will the weight of the fuel in the tank settle the car on its springs and tyres, the fuel will expand and contract and could bust something. You could slightly overpressure the tyres. Very important to take it for a decent run of 20 miles or so to charge the battery and give yourself the best chance of having some charge left in it when you get back. Switch everything off at its individual switch: radio, heater fan, interior light (totally off). Consider leaving a Disklok (£100 from Halfords) over the steering wheel.