Farming.
It's the only one of the three I do primarily to make money in the first place.
I fish mostly to fill out bundles, to potentially fish up some treasure, and a bit for cooking/gifts - I generally only sell fish to get them out of my inventory if I don't need them for anything else. And I mine mostly to get ore for bundles and upgrades, to kill monsters, to find treasures and to get the rewards for reaching deeper levels, and I generally don't sell ore at all - I just stash it.