Is this in ground? Or in a planter? Please show a photo of the site & surroundings.

Dwarfing rootstocks are generally pretty weak, especially certain apple rootstocks where it’s required to stake tree. Being in SoCal though I would plant any citrus you can find on flying dragon rootstock

3' is an extremely narrow width to accommodate any kind of tree, even dwarf ones. Columnar apples, like Scarlet Sentinel, North Pole or Emerald Spire, could work but may not be suitable for your climate. If the sun exposure is good, espaliered trees could work. Or even a grape vine trained along this length.

You will need something that remains pretty two dimensional or flat rather than a spreading crown that is typical to a 'standard' fruit tree.