Sairam Jena, With a single processor installed you would have to add the 3x 16GB dimms to slot A1, A2, and A3. Then place the 8GB dimms in slots A4, A5, and A6.  That will ensure that the channels match and will allow you to use all the dimms.  In regards to the speeds, it will depend on the dimms installed and their configuration.  Hope this helps clarify things. 
 Hi, I am using Dell PowerEdge R710 server,right now having 24GB of memory in that sever.Current Configuration is  I want to upgrade my memory.So I want to use 16 Gb memory.Could you please suggest me can I use 3 x 16 gb memory in other slots and what is the speed will support.  Thanks Sai 
Memory Upgradation.