My only problem with planting in pots of any size outdoors, especially in a warm climate, is that they need very frequent watering. When I lived in zone 5 and had a large deck with a lot of potted plants I watered morning and night.

If you use that 3' diameter pot for compost it should work if you keep it moist enough. I don't think I'd do potatoes in one tho. I had herbs, annuals, roses, and lavender in mine.

Altho my preference in pots is usually terracotta, the plastic/resin ones don't lose moisture as quickly so are easier to use outside.

One of our local organic experimenters and farmers tried growing potatoes in a 10-food diameter ring about 1 foot high and filled with nothing but cedar mulch. The potatoes were huge and extremely easy to dig in the soft mulch. Being on the ground he didn't worry too much about drying.