Every led series is powered by a 20mA costant current source with soft start. This improve the leds life and you can power them with a very wide voltage range. Min Input voltage is (nLed*vLed)+1= (2*3)+1 = 7 Volts
Warning , each smd pnp transistor works like a resistor. Each one dissipate P = (Vin-Vseries)*iLed = (15-6)*0.02 = 0.18W
BC858 pnp transistors are cheap and small. Max values are 250mW , 100mA and 30V
