combine text field based on value

I have one table with text column(varchar50) and 2 value columns(INT)

it looks like this:
c1, c2, c3
1, null, text1
2, null, text2
3, null, text3
4, 3, text4

What i want to do is to combine c3 on row 3 and 4 so the output would look like this:
c1, c2, c3
1, null, text1
2, null, text2
3, null, text3
4, null, text3\text4

I have tried to use CASE with no luck.

edit:
output like this is also fine
c1, c2, c3
1, null, text1
2, null, text2
3, null, text3
4, null, text4
5, null, text3\text4


Thanks in advance //Mr