Bro, I thought you already go through the beginner guide provided by Nikon? It's very well done, they give very good example with pictures showing how the image look when you change the aperture; There's also manual mode, which you can play with the shutter speed slider and aperture slider to experiment yourself how the image will turn out. Such great effort done by Nikon leh.

Anyway, for the picture you mentioned, it depends on several conditions, e.g. focal length, subject distance, etc. You will also notice at different focal length, the distance between the 2 subject will appear differently (at 18mm you can see the distance is further away, when you zoom to 50mm, the distance between the 2 subject becomes closer).

To get the 3 person in focus, you can try with smaller aperture maybe like example: 5.6? Then you should FOCUS on the center between both the subject and play around with the aperture; you can also experiment with the FOCUS, and see how the depth of field changes if you FOCUS in the front subject and also the rear subject.
If you focus wrongly (e.g. the front subject), then if you want to get the rear subject in focus, you will need very small aperture...
You can do testing yourself, put 3 rows  of books (example; OR your beloved Dam DAM = Gundam), and play with the focus point and the aperture to experiment yourself.
Also try with the shooting distance, try shooting close-up and slowly move further back, and see the difference... When you shoot closer, the depth of field is more narrow... also try different focal length...