How to create nodes automatically using title, fields, etc from some other nodes? I want to generate nodes for a content type using fields of some other content types. For example I have three content types A,B,C. All three content types have some common fields as well like Title, Category and Date. I want to create another content type D to hold only those common fields. 

Now here I want to generate content automatically for content Type D. I'm looking for a technique in which I can define that, "fetch data" from these common fields of A,B C content types to fill fields of content type D and generate nodes automatically for content type D.

Any suggestions about how to get this to work?
The Rules module is your friend here.

You may use it to create an entity (here a node) after saving a new content of type A B or C (or any other event). Then you can add actions (set a data value) to fetch field values from A, B and C for your new content of type D. The first action would be of course Create an entity, then set data values.

You may find a great set of tutorials about Rules at Learning the Rules framework series.

Here is a short example : after saving a new Drupal Article, create a new Drupal Basic Page with the Body extracted from the Article and an automatic node-title.

{ "rules_automatic_node_creation_based_on_another_node" : {
    "LABEL" : "Automatic node creation based on another node",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "ON" : { "node_insert--article" : { "bundle" : "article" } },
    "DO" : [
      { "entity_create" : {
          "USING" : {
            "type" : "node",
            "param_type" : "page",
            "param_title" : "Basic page from Article [node:title]",
            "param_author" : [ "node:author" ]
          },
          "PROVIDE" : { "entity_created" : { "entity_created" : "Created entity" } }
        }
      },
      { "data_set" : { "data" : [ "entity-created:body" ], "value" : [ "node:body" ] } }
    ]
  }
}