I have three pages
page 1 :through a submit button in page 1 ...
I want to be redirected in page2 in new tab and page3 in the same tab.
Is that possible?
Page1 contains :

            using (MemoryStream memoryStream = new MemoryStream())

                document.Save(memoryStream, false);

                memoryStream.Position = 0;

                Session["BinaryData"] = memoryStream.ToArray();

                ScriptManager.RegisterStartupScript(this, this.GetType(), "openPDF", "javascript:window.open('../../Service/Pages/Page2.aspx');", true);


page2: to be opened in a new tab

protected void Page_Load(object sender, EventArgs e)
           if (Session["BinaryData"] != null)

               Response.ContentType = "application/pdf";
               Response.AddHeader("Content-Disposition", "inline; filename=" + filename);
               ////Write the MemoryStream to the Response.

then page 3:
will be redirected in the same tab of page1

Thanks in advance

Could you use the response.redirect to open the page in the current tab and from there, using javascript window.open, make opening the new tab one of the first things your second page does. That's almost what you're doing now, just the flow is different.
In your current example it looks like you're not calling the script you set up (maybe, been a while since I did registerstartupscript stuff).