If I use Rare Candy to play Galarian Obstagoon, can I use its "Untamed Shout" Ability to place 3 damage counters on my opponent's Pokemon?
Yes, because it is still being played from your hand.
Source: Sword & Shield FAQ (2020-01-24), TPCi Rules Team (2020-02-06)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH1_119
SWSH1_119