I would allocate about 350w for each r9 280x card if you’re planning heavy overclocking.
350 * 3 = 1050 watt
350 * 6 = 2100 watt
Don’t forget about your motherboard, CPU, fans… so the one that is running these things and the 3 GPU’s should consider another 200-300 watts for that PSU:
So 750w power supply would run one card as well as the motherboard, cpu, hard drive, all fans.
So let’s go more conservative and have 4 GPU’s (will probably run cooler too):
350 * 2 = 700
350 * 4 = 1400
The setup for this would be 2 x 850w = 1700 w and will give you plenty of room for peripherals.
What this tells us is that alongside their similar on average performance, the GTX 770 and 280X also draw similar amounts of power under gaming workloads.