You cannot regulate voltage and current at the same time. You can regulate the current for LEDs.
You must understand that LEDs are not light bulbs, instead they are diodes.
Light bulbs are driven with voltage and they take their rated current.
But LEDs are driven with current and they set their own voltage.
LEDs have a RANGE of voltage so your "3.6V" LEDs might be 3.3V or 3.9V or anywhere in between. If you have three LEDs in series and are powered with 11.0V then they instantly burn out if they are 3.3V or they will not light if they are 3.9V.
So you connect only two LEDs in series so that their required voltage is from 6.6V to 7.8V and then their current is limited with a series circuit or resistor.
You must understand that LEDs are not light bulbs, instead they are diodes.
Light bulbs are driven with voltage and they take their rated current.
But LEDs are driven with current and they set their own voltage.
LEDs have a RANGE of voltage so your "3.6V" LEDs might be 3.3V or 3.9V or anywhere in between. If you have three LEDs in series and are powered with 11.0V then they instantly burn out if they are 3.3V or they will not light if they are 3.9V.
So you connect only two LEDs in series so that their required voltage is from 6.6V to 7.8V and then their current is limited with a series circuit or resistor.