Philippe,
Ha-Green; S2-Red; O3-Blue
I don't like the predominance of green that this combo produces due to the deeper data in the ha. I use a curves layer in the clipping mask to reduce the ha; and one to boost the S2. Then after flattening, I use selective color and make adjustments for the final hues (only after fixing gradients, background color casts by zooming in on the background and using select color range; then curves in appropriate channel). I use lab color mode and increase contrast to boost saturation. I am red-green colorblind so this may play a role in my results.
