We covered the top mattress with a white pack & play crib sheet. Pack & play crib sheets are a perfect fit for these mattresses because they were meant for the portable crib size.
For the other mattresses, I had to think outside the box because pack & play crib sheets didn't come in a lot of color options. I figured jersey would have a good amount of stretch to it and went over to the bedding section of Target. King sized pillowcases weren't too far off in dimensions with the mattresses so I thought it was worth a try.. and it worked!
The bottom two mattresses are covered using jersey knit king pillowcases. I used two different patterns. They come in a set of two and you'll need both to cover one mattress. You'll place each pillowcase in opposite directions over the mattress (one over the other).
Olivia loves sitting here and, occasionally, she lays each mattress on the floor and jumps around all three of them!
PS. These mattresses would also be good for triangular play tents! I wish I had figured that prior to DIY-ing my own (and sewing a cover for it).