I do not believe 3x4GB is a supported configuration for the DZ77RE-75K.
Your board supports (1 DIMM) in slot 1 or slot 3.
Your board supports (2 DIMMS, or 4 DIMMS) in Slots 1+2, or 1+2+3+4.
So I recommend adding a 4th 4GB DIMM, or remove one of the existing 3 DIMMs and move the remaining 2x4GB into slots 1+2.
See the diagram below for slot locations/numbers.
http://downloadmirror.intel.com/21240/eng/DZ77RE-75K_IntegrationGuide02.pdf