Subquery Inside Case?

Hello,

I am trying to do something like this:

select (CASE
WHEN tableA.col = 'a' then 'A'
ELSE (select table3.col1
from tableA, table 2, table3
where tableA.id = table2.id
and table2.id = table3.id )END ) as TEST
from tableA


But the problem is that the part in bold returns more than one row..
i want it to be select table3.col1
from tableA, table 2, table3
where tableA.id = table2.id
and table2.id = table3.id
for every value of tableA.col.

How do I do this?