Intel DQ67OW - Three Dimms; Example Dual Channel Memory Configuration with Three Dimms

Installing and Replacing Desktop Board Components
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Three DIMMs
If you want to use three DIMMs in a dual-channel configuration, install a matched pair
of DIMMs equal in speed and size in DIMM 1 and DIMM 3 of channel A. Then install
another DIMM equal to the speed and total size of the DIMMs installed in channel A in
either DIMM 2 or DIMM 4 of channel B (Figure 15).
Figure 15. Example Dual Channel Memory Configuration with Three DIMMs