Why is 3-chip forward current less than 1-chip in RGB LED LTRBR37G?
Power dissipation, the package can dissipate a set amount of heat, in page 4 there is a table showing the thermal resistance for 1 chip and 3 chip operation

in other words, you have the dissipation capacity of the chip to the package which limits the power to a single chip and then the resistance of the package which limits the power to the whole device