Grid Row

I am working on grid row. I would like to have 3 pictures in a row as on this website under the main slide: https://www.noa-noa-wuerzburg.de/
What is the dimensions of each picture and what setting should I use to fit the picture exactly in the square (it should also resize together with the resizing the browser window). What ever I do (stretch to fit, do not repeat, use different size of the picture) it shows or only the part of the picture or a small picture.
Thank you.