Shortest Query to Combine Count and Unique (Oracle)
Not that I recommend it, but:

select count(distinct ora_hash(col1)||'|'||ora_hash(col2)||'|'||ora_hash(col3))
from my_table
where cond1 = 'A'
and cond2 = 'B'
and cond3 = 'C'


A nested query seems like "the right answer" to me.