Nancy0623,  Having three dimms is what is causing the issue. This is for both processors, so match what we do on one to the other bank. . Try moving the dimms to the following locations. Place 1st dimm in slots closest to the center of the motherboard. From there place next dimms in every other slot. Meaning that you will have dimms in slots 1 (closest to center) 3 and 5 (5 being the second to last slot on edge of motherboard). The problem is that the 3 channels across the dimms must match and when you placed additional dimm in the server you had Channel A with 2 dimms and B with 1, hence the error.  Let me know if this helps. 
Hi, Dell! I'm currently using Poweredge C6100 server(4 nodes) and there has been a problem with configuring memories. Normally in one node, I place two 4GB memory chips on each processor, which is total 4 of them, and it has worked well. So I tried to add one more on each so that it configures total 24GB, and I put three 4GB memory chips on each processor. But the system didn't recognize and a blue screen with error messages came up. The modules are same size, and the memories are fine. What have I done wrong? and How may I solve this?
Dell Poweredge C6100 XS23-TY3 server: Memory configuration problem