ShowModalPopup opens in new window on postback

Ok i have used <base target=_self> in head tag of the page for, one of my other aspx page it worked. But
following is the senario, it is not working.
From page 1, i open modal dialog using following code :
 
var PageName = /*Page 2*/
var dialogArguments = new Array();
var query="TAPID=88&amp;/*Some queryString Variable here*/" ; window.showModalDialog("../"+PageName+"?"+query,dialogArguments,"toolbar:no;directories:no;location:no;status:yes; menubar:no; resizable:yes; scrollbars:yes; dialogWidth:1000px; dialogheight:700px");
Now in Page 2,
i do Server.Transfer(/*URL of Page 3*/);
Page 2 does postback..blah..blah... working fine...awesome.
Now, Page 3 opens in same popup window, just fine till here.
Now i have dropdown on Page 3 that do postback. or any other control on this page that do postback.
Same page 3 is ...