Well, I'd have to RTFM to be sure, but I don't think you can code a complex component ID into findComponent. I think you have to do a findComponent on pnreviewform, then run a findComponent on that to get msg3output.
Although in most cases, it's probably going to be easier to just bind the msg3output control straight to the backing bean using the "binding=" attribute.
Just because I take people to task here for overusing bindings doesn't mean that binding is useless. It just has its place, that's all.
Incidentally, my own personal recommendation is to avoid using raw HTML in JSF view definitions. You can use the panelGrid control in place of the raw table definitions. Same end result, but more JSF-friendly.
An IDE is no substitute for an Intelligent Developer.