Three Dimms; Example Dual Channel Memory Configuration With Three Dimms - Intel BLKDH67BL Product Manual

Installing and Replacing Desktop Board Components

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 14).
Figure 14. Example Dual Channel Memory Configuration with Three DIMMs
37