There are three basic issues with the Discovery 3 and sand/dune driving that you need to be aware of:

• Standard issue tyres are not capable of providing a sufficient footprint for the car, given it's weight:
• 235/70 17" tyres are not wide enough
• 18"and 19" tyres can probably not be deflated enough (I have 17" rims on my car, and this is a guess only)
  
• DSC inhibits momentum - the moment a wheel starts to slip, traction control brakes the slipping wheel. In sand one would like some slippage to occur.
• Suspension lowers above 50 km/h  - which can be a problem if you are making up momentum for a large dune
  

I achieved exceptional results with the following setup in Namibia:

1. Tyres 265/70 17" provides a large enough footprint - I have had it deflated down to 0.5 bar in front, and 0.6 at the back in extreme cases, but in most cases 0.9 in front, and 1.0 in the back will be sufficient.
2. Set Terrain response to Sand Driving
3. Low Range
4. Switch DSC off - this is very important.
   
5. Seatbelts on - for your safety, and the warning bell will distract you .
   
6. Raise suspension, and in cases where you suspect that you will exceed 50km/h, ask the front passenger to raise the suspension again the moment the speeds drop to below 40. (If the front passenger is incapable, do it yourself!
7. Accellerate smoothly, and build up speed as quickly as possible, without allowing the tyres to dig in.
8. If your car is automatic, let the automatic gearbox do it's thing - it does it better than you would in the heat of the moment!
9. If your tyres are very deflated - do not make a sharp victory turn at the top! The tyres might just pop off the beads...