If I attack with Wormadam's "Matron's Anger" and I have 3 parts of a V-Union Pokemon in my discard pile, does that count as three Pokemon for adding to the attack damage?
Yes, in this case "Pokemon" in Matron's Anger's attack text means "Pokemon Cards".
Source: Brilliant Stars FAQ; TPCi Rules Team (2022-02-24)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH9_77
SWSH9_77