8, 5 watt LEDs that run from a 12VDC source.
3, 18650 batteries at 3000mAh each.
I want to know the approx run time of the LEDs.
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Your load is 8*5=40W. Your source has 3A-h*12V = 36 W-h. In ideal case your LEDs will shine for 60*36/40 = 54 min = 0.9hr.
In reality, a 3,000 mAh battery has 3.7 * 3 *3 = 33W-h at best, since the average discharge voltage per Li-Ion cell is about 3.7V.
More, in best case the LEDs must be driven by switching type drivers, likely each drives two typical LEDS in series, and 4 chains. Switching-mode (the best!) will likely have 85-90% efficiency in 12:8 conversion mode, so overall your LEDs will run 60 * 33/40 *0.85 = 42 min. If you have passive ballast resistors to drive the LEDs, it will be quite less than 40 min.
The above assumes that the LEDs are driven to truly 5W each.