What we've found with this planter is that not all plastic 3" pots are the same height, and therefore do not uniformly fit below the top of the planters. If you prefer to not have a plastic pot showing, then we encourage you to find a plastic planter with height of 3". If you can't find a plastic pot of that sizing, the most common ones are 3 1/2".
Add some dimension and spray paint the plastic pots to create a colorful addition to your planter! We left the bottoms unpainted because the overspray makes a really cool gradient texture that we grew fond of.
Another simple way to style these planters is to stack them! We did this in our art installation. All of the plants that we tried to flip horizontally stayed in just fine, however loose, dry dirt tends to fall out. Keep your plant watered and the soil will stay more compacted together.