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.