Each "x1" for each slot is one lane. So an x16 slot has 16 lanes, an x8 slot has 8, an x4 has 4, and x1 has one. The x58 chipset has a total of 36 PCI-E lanes, which means that the active slots can have a total lane count of 36. This is why using three cards cuts the graphics slots down to 16/8/8; three x16 slots require 48 lanes which is more than the chipset can provide, but 16/8/8 requires 32, which is within its capabilities.
With your three graphics cards, you will be using 32 lanes in a 16/8/8 configuration. As for the other two cards, I'm not sure what the exact breakdown will be, but presumably they will make use of at least one lane from the southbridge, since the northbridge doesn't have enough lanes for the three graphics cards and your other two cards.
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