SQL> with table1 (col1,col2,col3,FUNC_col3) as (
  2  select 1,2,3,'COL1' from dual union all
  3  select 1,2,3,'COL2' from dual union all
  4  select 1,2,3,'COL3' from dual union all
  5  select 1,2,3,'COL4' from dual)
  6  select a.*,decode(func_col3,'COL1',col1,'COL2',col2,'COL3',col3) f from table1 a
  7  /

      COL1       COL2       COL3 FUNC          F
---------- ---------- ---------- ---- ----------
         1          2          3 COL1          1
         1          2          3 COL2          2
         1          2          3 COL3          3
         1          2          3 COL4 null