I’m not sure why your assuming 3 emitters since Sofirn wants 1. Anyway, I will use the L6 as an example as I’m more familiar with it. When it comes to heat generation, it’s directly connected to how many watts of power we are generating. The stock L6 pulls about 4.7A at the tail cap and that is basically too much to be sustainable and not burn you. Let’s say the voltage sag of two good batteries gives 8v. So 4.7 x 8 = 38 watts.
A single xhp70.2 would need to draw about 9A to generate 5,000 lumen. Let’s make the voltage sag a bit more and say 7.8v x 9A = 70 watts. That’s way too hot.
Now to get 5,000 lumen with 3 x 70.2 we would need about 6.6A. Let’s say the voltage sag is 7.9v x 6.6 = 52 watts. It can not sustain that. It will have to activate thermal protection and drop down in power.
I would estimate that we need about 30 watts (about 3.7A) to be both sustainable and to not burn your hand. 3.7A with a single emitter is about 2250 lumen. 3.7A with a triple emitter setup is about 2800 lumen.
So even though a triple 70.2 setup is more efficient, there is no way it can sustain 52 watts.
Currently there are only a couple of lights that can sustain 5,000 lumen. One of them is the Acebeam X65. You can get an idea of its size and heat sinking fins here.
It is really big! Much bigger than the light Sofirn wants to build.