Hey there! I totally get your situation - inheriting a 15-year-old production line with three different PLC brands is like trying to manage a team that speaks three different languages. The good news is that you can standardize without shutting down production for months. Here's the most pragmatic approach:

First, adopt a phased migration strategy. Don't try to replace everything at once. Start by creating a detailed inventory of all your PLCs, their functions, and how critical they are to production. Then, choose one brand to standardize on - Siemens or Allen-Bradley are usually the top contenders since they have better migration tools and industry support.

The key is to run new and old systems in parallel. You can install the new standardized PLCs alongside the existing ones, then gradually transfer control. This way, if something goes wrong with the new system, you can quickly switch back to the old one without stopping production.

For your specific mix, I'd recommend starting with the Mitsubishi FX units since they're typically simpler, then move to the Siemens S7-300s, and finally tackle the Allen-Bradley CompactLogix. Use available migration tools - Siemens has good migration paths from S7-300 to newer models, and there are third-party tools that can help convert between brands.

Plan this during scheduled maintenance windows or slower production periods. Document everything meticulously as you go - this will make future maintenance much easier. The whole process might take 6-12 months, but you'll avoid the massive downtime of a complete rip-and-replace approach while ending up with a much more maintainable system!