I am not sure I understand the question but this might be what you want:
select
sum(case when col3 <> '' then col1 else 0 end) as c1,
sum(case when col3 = '' then col1 else 0 end) as c2
from <table>
this will sum every row with non-blank col3 into c1 and every row with blank col3 into c2.