The fx-6300 is clearly the strongest of the three. Of course, you must overclock it for it to be a decent cost-effective solution.
The Phenom II is 4 cores on the cheap, but its also old & just not as good.
The AMD A10-5800k is what you buy if you want a gaming computer (excepting some CPU-dependent games) and
don't have the money to buy a video card. It is otherwise inferior in terms of raw performance and cost-effecitveness vis a vis the fx-6300.
You should also consider an i5-3470. With a cheap B75 motherboard it's usually about the same price as an fx-6300 + decent overclocking motherboard like a Gigabyte 970-ud3, and while it is locked (can only run at stock), a stock i5-3470 has very roughly equivalent performance to an overclocked fx-6300.
The 5800k is a 4300 with a GPU
While basically correct, if my memory serves the a-10 series doesn't have L3 cache, or not as much, or something like that, making the 4300 a little better than the 5800k in pure-CPU applications.