How to use single constant current source to power parallel LEDs safely and efficiently?
Different diodes are not completely identical and can not be connected in parallel safely. They should be connected in series, or each parallel diode should have their own current limiter circuitry.

If we drop the input current from constant current source so that it is equal to the maximum forward current of any one of them they can be connected safely, but current will divide and each LED will not get enough power and it will be dimmer.

source current &lt;= Forward voltage of a single LED 


Parallel LEDs efficiently glow if we make the source current equal to the sum of forward currents of all. 

source current = Forward voltage x number of LEDs


This has another problem. One of the LEDs might draw slightly larger current than other, become hotter, draw even more and become even hot until it will fry itself. This propagates until all LEDs fry. 

So there is trade-off. We can use slightly larger current source than any individual LED can handle and connect LEDs in parallel. It should be smaller than sum of forward current of all LEDs in parallel.

Forward Voltage of a LED &lt;= Source Current &lt;= Forward Voltage x no of LED


This way LEDs will be brighter and have longer life. We have to set source current near to lower limit for longer life and near to higher limit for brighter light. 

We should connect a good quality heat sink to all LEDs so that one will not be hotter than other to prolong its lifespan.

The best alternatives are


use as many current limiter circuits as number of LEDs in parallel without changing voltage
use single current limiter and connect LEDs in series by increasing the voltage