Concatenate multiple columns automatically [duplicate]

I have three columns

A    B    C
D    E    F
G         H
          I

I want to write a formula such that it will combine the three columns, one on top of the other such that my fourth column is:

A
D
G
B
E
C
F
H
I

Adding a z at the end of the second column would automatically yield:

A
D
G
B
E
z
C
F
H
I