Please correct me if i am wrong.
Lets assume my PRM hits 100 damage. I weared an item that has +%20 fire damage then my PRM will hit 100 + (20 ÷ 3) = 106.66?
If i wear +%20 elemental damage instead of fire one my PRM will hit 100 + 20 = 120?
Thanks in advance
It is correct.
Assuming you do not have additional conversion happening, here. The +%fire will apply to 33 damage, while the rest is unaltered.
The +% elemental will apply to all 3 sources, thus all 100 will see their damage modified.
If you have conversion fire -> cold or equivalent, then depending on when the conversion occurs, things can get more tricky.