Is this a good plan for a mini goldfish pond?

This sounds great,you don't even need to replace the Water if you get a couple of bunches of weed and a marginal plant or two,just clean out the filter every month or so.How big is it,because if it is larger than, say,40 gals you might squeeze in 3,that said comets are quite big.If your pond is in the shade under some trees,however,stretch some netting over the top to catch leaves-I have a pond in semi-shade under some creepers which is fine.Good Luck!