I need help. If I have data of varying lengths in 3 columns like :

a1 b1 c1
a2 b2 c2
a3 b3 c3
b4

in the output I need one column like this :

a1 b1 c1
a1 b1 c2
a1 b1 c3
a1 b2 c1

and so on... It would be great if I could get some help with this.