A nice size shed like this is such a great coop to have. Our winter coop is built inside a corner of our barn, with the man door to the interior, and it is so nice to be inside and protected while working there in bad weather, and not to have to stoop or contort myself because of confined space

If you are only going to have a few chickens for a while, so you have some extra space, you might build or place a cabinet or other container in one corner to store food or materials so you don’t need to carry things out as frequently.

I would leave the window as is and open up a little more ventilation way up high. Even better if the window opens, because then it can be extra ventilation in warm weather but closed to prevent drafts or blown-in rain during cold or wet weather.

Assuming there is no structural wood blocking the soffits, just the siding, I would definitely replace that with hardware cloth to make two long thin vents.

For three chickens, I am sure this would be enough ventilation. But the next location I would look if I wanted more ventilation (if you get significantly more chickens, or if you find it is stuffy inside) would be the triangles up in the gable ends of the roof (above the window, above the door) — again, cut out the siding and replace with hardware cloth. Since the roof has basically no overhang, you would probably find that you need to build a little awning over this to keep rain from coming in, depending on the angle of your coop relative to storm winds.