How much damage does Diancie's "Diffuse Reflection" attack do if I also have an evolved Pokemon with a Reversal Energy providing three energy?
The damage is calculated based on the number of Special Energy CARDS, not the amount of energy the card provides. So in your example it would only do +40 damage for Reversal Energy, not +120.
Source: TPCi Rules Team (2024-09-05)

Relevant cards

Note: All applicable cards may not be listed below.
SV7_86
SV7_86
SV2_192