Heat dissipation Aluminum PCB
No, it means that each junction runs 18C hotter than the heatsink(PCB). 

You need to know the heatsink's thermal resistance to air, and the total power you're dumping into it, and the air temperature, to calculate the heatsink temperature. Then add 18C to that, to get the chip temperature.

If the heatsink is just the PCB, its resistance will be high and its temperature will be high. It's difficult to calculate (as in, read a thermodynamics book first), maybe possible to simulate with professional design tools, but easiest to measure (bolt a power resistor and some thermocouples to it). But there's no point, you won't like the answers.

Practically this PCB isn't the heatsink, it just transfers the heat to the real heatsink (big lump of metal). 

Now you need to know the real heatsink's thermal resistance to air, and the total power you're dumping into it, and the air temperature, to calculate the heatsink temperature. This thermal resistance will be in the heatsink's data, if you buy a commercial one. Otherwise, see above re: calculation, simulation, measurement.

But the PCB will run hotter than the heatsink : you also need to know its thermal resistance in transferring heat from chip through PCB to heatsink.
Again, probably easiest to measure. 

This is where you need to know the total power from all LEDs - if it's 1C/Watt, it will add 6.6C or 13.2C for 3 or 6 LEDs at 2.2W each.

Now, calculate the heatsink's temperature. Add the temperature due to the PCB. Then add 18C, and that's the chip temperature.