var query1 = select * from table 1
var query2 = select * from table 2
var query3 = select * from table 3


GridView.DataSource = query1;
GridView.DataSource = query2;
GridView.DataSource  = query3;

var aggregateQuery = query1.Concat(query2).Concat(query3);
GridView.DataSource = aggregateQuery;

DataSourceSelectArguments dss = new DataSourceSelectArguments();
        SqlDataSource sds = (SqlDataSource)YourControl.FindControl("your datasource control");
        if (sds != null)
        {
            DataView dv = (DataView)sds.Select(DataSourceSelectArguments.Empty);
            if (dv != null)
            {
                DataTable dt = (DataTable)dv.ToTable();
                if (dt != null)
                {
                  myGridView.DataSource = dt;
                  myGridView.DataSourceID = null; //won't be needed
                  myGridView.DataBind();
etc