Specifications of the rear hub: 
Brand: Sturmey Archer
Model: X-RD3
Color: Silver
Number of spoke holes: 36GTS
Type of brake: Drum Brake 70 MM
Number of gears: 3S
Type of axle: Fixed axle 163 MM
Axle included: Yes, with nuts
Type of bearings: Ball bearings
Average weight (GR): 1580
Intended use: City
 
Data for spoke length calculations using the Sapim spoke size calculator: 
Total length of the hub (MM): 118
Non-gear side (MM): 18
Gear side (MM): 32
Hub diameter non-gear side (MM): 89
Hub diameter gear side (MM): 89
 
Link to spoke calculator: www.sapim.be/spoke-calculator