How the TL/SP works internally in AIX, the SP is nothing more than a collection of rules:
fileset x.y.z should be at version >=126.96.36.199 (if installed at all)
fileset a.b.c should be at version >=188.8.131.52 (if installed at all)
These rules can be checked via instfix command. If at least one rule is not fulfilled, it means you don't have this SP level, but a lower one. For example, if you will now install some old fileset, you might notice that "oslevel -s" will show lower SP level. This is by design.
So really, what you ask is probably how to lower some versions of filesets.
The first thing I would try is to list uncommited filesets in smitty. These filesets have updates that may be rejected (i.e. the system still has the original files, and can revert to earlier version). Only when you commit the filesets it removes the original versions of files. When updating AIX the default smitty setting is, unfortunately, to automatically commit.
Next thing to try is restore the backup.
The last resort is to try to reinstall all filesets to the original version using original installation media. This will probably preserve contents of various configuration files. I haven't done it and it is surely not a standard operation. It might be even impossible because of complicated set of dependencies between system filesets.
May I ask what went wrong with TL02 SP3?