group by a string filed (concatenate the string field)

Here is my PL/SQL query

select tbl1.col1, tbl2.col2, tbl3.col3
from tbl1, tbl2, tbl3
where tbl1.col1 = tbl2.col1
and tbl2.col2 = tbl3.col2
group by
tbl1.col1, tbl2.col2, tbl3.col3

here is the output

col1             col2                 col3
1                 123                  val1
1                 123                  val2
2                 234                  val3
2                 234                  val4
2                 234                  val5


i want to see the output as

col1             col2                 col3
1                  123                 val1-val2
2                  234                 val3-val4-val5

in other words i want to concatenate the col3 field

hope i was clear with the description..... please help me out

Thanks