I am going to make a 12.6v battery from 3 x18650 battery and going to use 3s bms board.Can somebody tell me where can I solder the leds to indicate when the battery is fully charged and when is drained?
As far as I am aware there is no simple LED circuit that will do what you want.
You can do it with a pair of analog comparators which is available in one IC package.
You could use an LM3914 dot/bar display driver to give an indication of the voltage.
You can use as many LEDs as you want the desired resolution (e.g. discharged, half-charged, full charge).