If you're cleaning handguns and handgun bores, I've yet to find anything that works better than Hoppe's #9 or Ed's Red, though other products may work as well or almost as well. I've tried so many cleaners over the years, I can't remember them all, but most were unimpressive.
Carbon is not much of a problem in handgun bores and it's not difficult to remove with most cleaners and a good bronze brush. A carbon cleaner is not necessary, but it won't hurt anything.
Carbon buildup in rifle bore is completely different, but I will assume you're talking about handguns here.
I've found 3 in 1 products to be compromises at best. They're probably most useful for providing an adequate level of lubrication for most purposes. For cleaning, they don't work as well as a straight solvent, but are okay for minimal cleaning if you have nothing better. As for a preservative, they work, but no better than many other light oils on a soft cotton cloth.
However, many get by with 3 in 1 products so they apparently have some usefulness.
