Here is my calculation:
Given: 3 pcs. Luxeon Rebel Uf=3.5V (max), If=700mA
LM1086 dropout voltage = 1.5V
reserve = 2V
Vref=1.25V
Voltage demand LEDs Vtot = 3X3.5V, Vtot = 10.5V
dropout voltage = Vdo = 1.5V
reserve = Vres = 2V
total minimum input voltage = (Vtot+Vdo+Vres) 14V (zener diode not required)
Radj=Vref/If = 1.25V/0.7A, Radj = 1.785Ω, next standard value 1.8Ω.
Power dissipation Radj = VrefXIref = 1.25VX0.7A = 0.875W - use a 2 or 5W resistor for least heating.
For LED cooling refer to the datasheet.
Boncuk