I take no credit, source - http://www.xbox360iso.com/battlefield-3-installation-t485028.html?p=3212038&posted=1
I only changed the parts to horizon, which makes it 10x easier
After extracting both disks into ISO’s, use EXISO to extract disk 1 to XEX folder format
In disk 1 ( which you just extracteed ), search through the content folder until you get to the 1.5gb file, this is the HD content pack, drag it into horizon in the device explorer and let it do its work
Download the 166MB title update and drag it into horizon, let it do its work
download it here - http://marketplace.jqe360.com/getfile.php?file=44429
Use ISO 2GOD to convert disk 2 to GOD format, then play with the GOD, it should work.
Im testing this now im pretty sure i didnt miss anything, so yeah