If you've got a new one on hand, at least put a new one in #3... I had one that was stuck and gave me flashbacks to your mess from many years ago; that one was all carboned up and I really had to spend a fair amount of time reaming it to get it cleaned out. Don't be shy on the reamer.

As for changing them all, I did but I'd have really regretted it if I managed to break a perfectly functioning glowplug in my desire to "do the job 'right'". Almost certainly the right thing, but I'd get the engine HOT first, and might even try to crack them all as quickly as I reasonably could. Number 6 gave me the most hassle, and the engine was really cooled off before I got to that one. Given how much carbon it had, I suspect it was going to be a ***** no matter what, but being hot would have been helpful I think.

If you get one that's really stuck, nothing says you can't leave it in there, reassemble the others, start the engine and heat it BACK up.