Hey there! I totally get your situation - inheriting a 15-year-old mixed PLC system with budget constraints is a classic plant manager challenge. Here's my strategic roadmap for your 3-year phased approach:

First, you should know that Siemens S7-300 PLCs are actually being phased out by 2025, so this modernization is more urgent than you might think. Here's my recommended approach:

Year 1: Assessment & Foundation

Start with a complete system audit - document all your S7-300 and Mitsubishi FX PLCs, I/O counts, and criticality. Create a risk assessment matrix. Focus on securing spare parts for the most critical systems first. This is also the time to standardize on a single platform - I'd recommend going with Siemens S7-1500 series as your target since you already have Siemens expertise, and they offer migration tools for S7-300 systems.

Year 2: Pilot & Training

Pick your least critical production line or section for a pilot migration. This lets your team learn the new Siemens TIA Portal software and S7-1500 hardware without risking your main production. Use this time to train your maintenance and engineering teams. For the Mitsubishi FX systems, consider using protocol converters or gateways to integrate them into the new Siemens ecosystem temporarily.

Year 3: Full Rollout

Now migrate your most critical systems. Siemens offers migration services that can help convert your S7-300 programs to S7-1500. For the Mitsubishi systems, you'll need to rewrite the logic in Siemens TIA Portal, but you can do this in sections to minimize downtime. Consider implementing a modern HMI/SCADA system like Siemens WinCC to unify your visualization.

Key tips: Always maintain production capability during migration, use simulation testing before cutovers, and keep legacy documentation updated throughout the process. This phased approach spreads costs, minimizes risk, and builds internal expertise gradually.