You can always boot, using the rescue CD (the original installation CD) and choosing rescue.
However, appending the "3" (without quotes) to the kernel line should have done it. You might also temporarily remove the rhgb quiet options, so that you can see if something is going wrong during boot.
It's possible that the new /etc/inittab changes have gotten intrusive to the point where they override boot options, but that seems unlikely. I think I would try to do the following.
When you get to the part where you can edit the kernel line, remove the rhgb quiet at the end, which should help you see what's going on during boot.
Then, allowing a space after the final word in the line, type "single" without quotes.
That should boot you into single user mode.
At that point, you should be able to edit /etc/inittab and change the default level 5 to 3. You have to change it, apparently, commenting out that line and putting in an identical line with 3 instead of 5 in it doesn't work anymore. (So I've heard, that's untested personally.)
Then, after editing it, if you type exit, you should be able to boot into runlevel three. If, after typing single at the kernel line doesn't work, though, then use my original suggestion of using the install CD, choosing the rescue option, and then, when it mounts the system under sysimage, go in there and make the suggested changes to /etc/inittab.
Do NOT PM forum members with requests for technical support. Ask your questions on the forum.
"I don't know why there is the constant push to break any semblance of compatibility" --anon