There is no legitimate or safe “Honda ECU 3.5 5.2 download” available to the public. What you’re likely referring to is either:
Software HondaECU from MCU Innovations. A collection of demo versions software from MCU Innovations (https://mcuinnovations.com/). HondaECU Release 3.5.5.2 - MCU Innovations honda ecu 3.5 5.2 download
Compatible with standard J2534 interfaces (like Mongoose or Tactrix). You can find the installer on GitHub via AutotronicCommunity or through dedicated tuning repositories. There is no legitimate or safe “Honda ECU 3
If you’re referring to for a Honda 3.5L engine (e.g., J35 series in Odyssey, Pilot, Accord), legitimate downloads are only available: J35 series in Odyssey