Is a CP3 conversion a good sign on a used diesel truck?

A CP3 conversion can be a good sign. It can also be a reason to ask more questions. The good version is simple: the owner knew the platform, used quality parts, had the work done correctly, kept receipts, and the truck runs cleanly with no fuel-pressure issues. That can reduce one known risk on some trucks. The bad version is harder to spot. Poor installation, bad tuning, incomplete parts, fuel leaks, rough running, or no documentation can turn a useful upgrade into a problem. Also ask why it was done. Preventive work is different from a conversion after a fuel-system failure. Before buying, ask for receipts, installer details, tune information, mileage when installed, and any fuel-system repairs done at the same time. Then inspect it like any other used diesel. A CP3 conversion helps only if the rest of the truck is right.