I use MSI Afterburner for doing exactly what you are talking about. There is a setting for synchronizing cards that are alike, but you want to make sure that is not selected. Then there is a drop down arrow beside where it says RX 580 and you can select each of your cards to adjust one at a time.
For overclocking you will want to increase MHz in small amounts until you find a stable limit. Then simply slide the voltage all the way down to -100mV. Assuming you are mining you can underclock the Core clock to use less power while overclocking the memory to get more hashes.