Edit photo from gallery with CSS

Hello,

I would like to make a custom post with a gallery at the top. I also have a simple lightbox plugin installed so I can click through the photos.

However, only the first 5 photos of this gallery should be displayed.

They should be formatted as follows.
First picture 33% width and about 600px height
Remaining pictures 33% width and 300px height each

Img1 | Img2 | Img3
Img1 | Img4 | Img5

All other images should be hidden.

My idea was to use a Gallery (to make Lightbox work), and format the single images here with CSS.

Unfortunately I can't manage to control the single images of the gallery in CSS.

Example (does not work):

img.galleryclassname:nth-of-type(1)
{
heigh: 600px
}
img.galleryclassname:nth-of-type(2)
...
..
..

img.galleryclassname:nth-child(n+4)
{
display: none;
}

I hope you can help me.
Greetings Stefan