Actually, 3 chickens is appropriate for the size of your coop

The main issue with the tiny coops is that people believe the number of birds that the manufacture says will fit. I would consider adding a roof vent or adding some more ventilation up at the top on the side under the run cover, especially if you live in an area with hot summers or cold winters - extra ventilation is needed in both situations.
As
@21hens-incharge pointed out, how your soil drains will make a huge difference. Since there isn't much of a lip around the bottom of the run, I would try to raise your set up on pavers no matter what. We have both sand and deep litter in our run -1/3 sand in the original run and 2/3 deep litter in the expanded area. There are advantages to both types. Our soil drains well, so both areas drain well. The sand needs to be cleaned at least once a day, more often is even better! I zip tied a piece of hardware cloth to a stall rake so I could clean under the raised coop and sift the poop from the sand. The sand gives a place for dust bathing, but compacts quickly and must be broken up with a rake or hoe.
The deep litter area is nice because I don't have to do anything besides periodically add more dry material. We have a mix of wood chips, pine straw, hay and pine shavings. The chickens spend equal amounts of time in both areas, but prefer dust bathing in the sand