Specifications of the rear hub: 
Brand: Sturmey Archer
Model: RX-RD3 ROTARY
Color: Silver
Number of spoke holes: 36GTS
Type of brake: Drum Brake 70 MM
Number of gears: 3S
Type of axle: Fixed axle 185 MM
Axle included: Yes, with nuts
Type of bearings: Ball bearings
Average weight (GR): 1413
Intended use: City
 
Data for spoke length calculations using the Sapim spoke size calculator: 
Total length of the hub (MM): 135
Non-gear side (MM): 21
Gear side (MM): 47
Hub diameter non-gear side (MM): 90
Hub diameter gear side (MM): 90
 
Link to spoke calculator: www.sapim.be/spoke-calculator