If I use Rare Candy to evolve Charizard ex, can I use its "Infernal Reign" Ability to search for and attach up to 3 Basic {R} energy?
Yes, because Charizard ex is still being played from your hand.
Source: TPCi Rules Team (2024-06-27)

Relevant cards

Note: All applicable cards may not be listed below.
SV3_125
SV3_125
SV1_191