Hi kankon,

Since you are writting SQL query you have to use AND operator and you need to convert the column value to DATETIME for compare.

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindGridView();
    }
}
  
private void BindGridView()
{
    string str = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(str))
    {
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "SELECT * FROM Table_vacation WHERE (namevacation = @Con1 OR namevacation = @Con2) AND CONVERT(DATETIME,datestart) >= DATEADD(MONTH, -3, GETDATE()) ORDER BY [ID] DESC";
            cmd.Parameters.AddWithValue("@Con1", "اجازة دورية");
            cmd.Parameters.AddWithValue("@Con2", "اجازة دورية للاشرافيين");
            cmd.Connection = con;
            using (SqlDataAdapter da = new SqlDataAdapter(cmd))
            {
                using (DataTable dt = new DataTable())
                {
                    da.Fill(dt);
                    this.gvFiles.DataSource = dt;
                    this.gvFiles.DataBind();
                }
            }
        }
    }
}