Installing software to a third partition is a waste of time to me. If you reinstall windows you have to reinstall the software anyway because registry entries are gone.
Joe
Since that appears to be a comment to my post above, note I said above the only thing on the 3rd partition is the page file.
But at any rate, your comment is not really true for most applications. If you take Office, for example and you previously had it on a 2nd, or 3rd partition partition, or perhaps the 3rd partition of a 2nd drive, when you restart the install process if it does not find the previous installation you simply point the installer to the previous location where it will find the installed files and simply make the necessary modifications to the Registry. It will even then keep all your customizations, and saved files in tact and takes a few seconds total, as opposed to a complete install - AND, it will have all the critical updates installed.
Also, many programs do not "install" they are simply saved to disk, and accessed by a shortcut in your Start Menu, Desktop, or Taskbar. In those cases, all you need to do is create a new shortcut - you don't have to reinstall anything.
Reinstalling Windows is always (or should be always) a last resort effort because that instantly puts you months, if not years behind in critical security updates. So while it is good to plan for it, it is not normal, or typical for Windows (since XP) to ever need a complete reinstall. So my point is, since a complete reinstall is such a rarity, I see little point in having that remote possibility dictate everything else, including my everyday activities.
Must admit I keep op sys, installed apps and associated files and settings all on the system drive. It seems more sensible to me to have installed programs installed on the same drive as their associated dll's and registry entries.
If you are going to keep all installed applications, associated files and settings on the boot drive, I see no reason to partition at all. And considering the reliability of today's drive, and the ease of keeping a current backup handy, that's not a problem. Note if you keep all your documents and saved data somewhere under My Documents, it is a simple matter of copying the whole folder to a backup location.
As to the comment about DLLs on the same partition, that to me is not important. The system does not care if a DLL is located at c:\program files\xyz or d:\utilities\xyz. Also note if you are fortunate to have two physical drives, you can improve performance by placing your apps on the second drive as now you have one head gathering Windows information while the other head is gathering application information. Or one head can be reading while the other is writing. With all on the same drive, regardless if partitioned, the single head can only be in one place at once, and perform one task at once.