WordPress login through iframe

I apologise first if this has been answered already, but i have searched, and even though it was asked, it was never answered.
I have my main site here: http://www.krillmeed.com/index.html As you can see it runs on iframes. Even though the sites like the ecards which is here: http://krillmeed.com/ecards/ And even a wiki, on a different domain: http://lcarsmemoryalpha.com/wiki/index.php?title=Main_Page Users can login through the mainpage no problem.
Except for the Blog, which is situated in this folder: http://krillmeed.com/blog/ Though it is on the same domain as the mainsite frames, i believe that 3rd party cookies are not allowing users to login unless they go directly to the blog.
I have read that this code could help: `header(‘P3P:CP=”IDC DSP COR ADM DEVi TAIi PSA PSD IVAi IVDi CONi HIS OUR IND CNT”‘);
` As i am told

Just add this to the start of your page (every page where cookies will be written):

Would this work? If so, were will i add it?
I know iframes are frowned upon these days, but aside from search engines, i like the design and it works. Any help would be appreciated. Thank you