Assuming you could get those cards working for mining purposes (and I don't see why you couldn't....probably going to be a nightmare to work with though), you certainly don't want the "cheapest" PSU available to you. Incidentally it would probably be:
http://www.outletpc.com/vq9179-solid-gear-sdgr-900e-atx-900-watt-power-supply.html?utm_source=vq9179-solid-gear-sdgr-900e-atx-900-watt-power-supply&utm_medium=shopping%2Bengine&utm_campaign=pcpartpicker&utm_content=Solid%20Gear%2B-%2BPower%20Supplies%20%3E%20ATX%20Power%20Supply
2x 6+2 connectors
2x 6 pin connectors.
Should be sufficient for an RX 480 (8pin), 260X (6 pin) and a 7970 (1x 8 + 1x 6).....right? :lol:
A $60 "900W" PSU sold at outlets like...... Walmart. No reviews online period, let alone anything reputable. I'd be inclined to keep one of these guys handy:
https://images.lowes.ca/img/x/9257/029054751027_ca.jpg
Not ideal for a mining rig with 24/7 usage! (and looking at it, it's rated as a 750W unit with 62A on the 12V rail.....apparently).
65W PSU, 270W 7970, 150W 480, 115W 260X..... plus the balance of everything = ~650W or so.
Given 24/7 usage of a mining rig...... I'd want to be running <50% of it's rated capacity. So you're looking at 1200W+/.... and those don't come cheap. With 600W+ constant usage, you want the best efficiency available too (to mitigate the power bill some).
PCPartPicker part list /
Price breakdown by merchant
Power Supply: Corsair - Professional 1200W 80+ Platinum Certified Fully-Modular ATX Power Supply ($214.99 @ Newegg)
Total: $214.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-06-08 17:19 EDT-0400
Incidentally....... do you have access to extremely cheap/free energy? (Solar etc)