About [Ad Inserter]

I see 8 rows and 3 columns.

Do you mean inserting between articles in the row or between rows?

Yes.
The middle of the vertical second row.
The middle of the fifth vertical row.
The middle of the eighth row in the vertical.

I’d like to display advertisements here.
Also please tell me the way to achieve it.

But rows are horizontal?

I still don’t understand where do you want to insert.

Desktop is
In the middle of the second row from the top
In the middle of the fifth row from the top
In the middle of the eighth row from the top
I want to display advertisements on.

https://bintan2ch.net/wp-content/uploads/2018/02/トップスクショ.jpg

I would like to place advertisements on the part surrounded by black.
※ This is the desktop version.

I see.

So this means that you would like to insert ads at those positions and the posts will shift so there will be another row.

You can try with automatic insertion After HTML element.

For the second row try:
.three-columns group .imag:nth-child(4)

Make sure the element inserted will be floating (as existing images float: left;).

However, this may be tricky and requires knowledge of CSS selectors.