If you only want to run 3 GPU's, you can get away with a cheaper board. I'm sure you can get cheaper options from ebay.
Motherboard:
ASUS M5A78L-M LX PLUS AM3+ AMD 760G Micro ATX AMD Motherboard 3 PCI-E slots for 60 bucks
Power Supply: I won't fight you against the Seasonic. Many people seem to have their own preferred brand. TBH, if you're mining, make sure it's at least 80PLUS Silver. Try to go for 80PLUS Gold though. Also single rail is a big plus, it means less figuring out load balancing. Your power supply choice meets both of those. I do not believe your system will consume much more than 600 watts with stock volts on the video cards though. I have 4 5870's undervolted and they draw 550 watts on 83% efficient power supply. Anyways, if you don't think you will expand, look into getting a lower capacity power supply, maybe 750 if it saves some money. Also, if you think you'll run out of PCI-E power cables, I find these are amazing.
Athena Power Model CABLE-EPCIE1628 16" PCI Express 6-Pin to Dual 8(6+2) Pin Linear Extension Power Converter It splits a 6-pin PCI-E power connector into 2 x 6+2 power connectors.
RAM: You can get away with 2 or 1GB of budget RAM. Dedicated mining on Win7 with 1GB of ram will work fine.
CPU: Overkill. Get a Sempron 130/140/145, save yourself some money and electricity.
HDD: Windows 7 will not install onto a USB flash drive (at least in my experience). You will only need about 15GB to be "safe". You can also look into finding a SATA to SDHC card adapter and install your OS onto an SDHC card. It will save you electricity.
Adapter and
16GB SDHC card $19 for both