I am not sure what you are planning on running for settings, but you are not going to need 3 7970s. IMO by the time you may need that power, a single 9970 will perform better than the crossfire setup. If you really must upgrade to a board with 3 slots, bear in mind that the 1156 and 1155 system CPU's control the 2 PCI-e express lanes directly for the graphics cards, at the moment you only get PCi-e spec 2.1 x16 slot, or 2x 8 slots, the extra bandwidth from the 3d slot has to come from the SATA/ USB bus, so you will lose performance overall if you run with 3 cards with any motherboard.
As you said, the 1366 and 2011 systems have more pcie lanes, so that isn't a problem with them, although they cost quite a bit more :)
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