Congratulatins n getting HF3 out, looking good!
I was trying compositing with OpenEXR to see how that went and I think I've picked up a math bug (or I'm being stupid I've been working with Nodes for ages instead of layers :-) ) but I think I have this set up correct.
In the attached example I've included just one colour images to try and keep it clear.
If you add two layers together both with the same example AddInput0001.exr file which just contains RGB 0.2, 1, 1 then the result should be RGB 0.4, 1, 1 which I've included for referance that I created in Blender to double check my sanity (AddResult0001.exr)
If you look at the example file I linked assuming I have it set up correctly :-) I get a result of RGB 0.93,1,1 when I do the same operation with layers in Hitfilm I noticed this as I wondered why some of my tests were blowing out.
If you look at that as HSV interestingly the correct output should be H 0.5, S 0.33, V 1.0 where from Hitfilm I get H 0.5, S0.031, V1.0 so it looks like it might just be a decimal point out in the code somewhere.
I had a quick look at the rest of the blend modes and they appeard to work as expected but I didn't look in detail
Hopefuly I haven't just set this up worng in Hitfilm!!