Grab images from left to right (not top to bottom) for Mobile stacking

I have a selection of 6 images on a page on my website. 3 on top and 3 on bottom.

Like this:

1 2 3

4 5 6

When they stack in mobile, they stack

1

4

2

5

3

6

How do i get them to stack as below:

1

2

3

4

5

6

I hope this is clear. 

Thanks!

Adam

Hi,

First, add 1, 2, 3

Next, add 1 Line Block under

next, add 4, 5, 6

Next, remove Line Block

They will stack correct on mobile