string mysql = "select * from carinfo";
                m_dataadapter = new SqlDataAdapter(mysql, m_connection);
                m_dataadapter.FillSchema(m_ds, SchemaType.Source, "carinfo");
                m_dataadapter.Fill(m_ds, "carinfo");


                mysql = "select * from cardinfo";
                m_dataadapter2 = new SqlDataAdapter(mysql, m_connection);
                m_dataadapter2.FillSchema(m_ds, SchemaType.Source, "cardinfo");
                m_dataadapter2.Fill(m_ds, "cardinfo");

                mysql = "select * from userinfo";
                m_dataadapter3 = new SqlDataAdapter(mysql, m_connection);
                m_dataadapter3.FillSchema(m_ds, SchemaType.Source, "userinfo");
                m_dataadapter3.Fill(m_ds, "userinfo");

string mysql="select * from cardinfo inner join carinfo on cardinfo.ID=carinfo.ID inner join userinfo on cardinfo.userID=userinfo.userID";
m_dataadapter = new SqlDataAdapter(mysql, m_connection);
m_dataadapter.FillSchema(m_ds, SchemaType.Source, "carinfo");
m_dataadapter.Fill(m_ds, "carinfo");

------解决方案--------------------
建议用 DataSetHelper ，里面支持对DataTable的 join,group ..

------解决方案--------------------
自己在DataSet中新增加一张表，然后通过Ado.net表达式实现，这样不需要管理数据同步，应该是比较好的一种解决方式，
http://www.microsoft.com/china/MSDN/library/data/dataAccess/ADONETEXP.mspx