SQL & PL/SQL

Say for eg: my data is like this

--order by col3
col1 col2  col3
1       5     1
1      10    2
2      2      3 
2      5      4
1     15     5
I hav to get o/p as
col1  sum_col2
1       15
2       7
1      15
I have to group by preserving the order
Pls give me suggestions..

jeneesh