Baner Garden Baner Garden N87 4-Piece Outdoor Veranda Patio Garden Furniture Cover Set with Durable and Water Resistant Fabric (Grey)

Q: Does rain water slide of the covers or accumulate and create puddles on top of the chairs and loveseats?

We suggest you put some round things to raise the cover up, so that the rain water can be flow down through the line.

Q: Will these cushions fit the Christopher Knight 4 piece set?

I suggest you use the measurements.

Q: I am looking for the same set but to cover a regular 3 person couch. Do you carry?

You'd better measure the couch, the longest cover is 51inch

Q: Do these covers have a vent to protect from humidity which causes mildew and mold?

They have no vent. But they have distance from the ground, and we suggest you put some round things to raise the cover up, so that the water and debris can be flow down through the line. The raise part can also provide airflow between cover and furniture.