Sounds like this refers to photography. It means that the camera was not looking directly at the subject, but was positioned 45 degrees from it.

“what you put in the main part of the photo” –

The main part of a photo is the foreground or the primary subject/s.

I’ve never heard of “three-quarter on you” but if an idea comes to me, I’ll add it here.

When shooting portraits or drawing – it is HIGHLY recommended to do so in the 3/4 profile. This shows depth of field and makes sure that the shadows show the roundness.

I have included some examples.

this may have been spoken in reference to a picture or series of pictures, taken at an angle which made it appear that one person was further afront from the rest of the runners that he/she actually was.