Using a standard round 3" galvanized HVAC pipe outside your home is not recommended. Galvanized HVAC pipes are typically intended for indoor use and are not designed to withstand outdoor weather conditions. When exposed to the elements, galvanized pipes can corrode and deteriorate over time, leading to potential leaks or failures.
For outdoor applications, it's recommended to use HVAC-specific materials that are designed to withstand the outdoor environment. One common option for outdoor HVAC ductwork is insulated flexible ducting, which is constructed with materials that are more resistant to weather conditions and provide better insulation.
If you need to install ductwork for an outdoor HVAC system, it's best to consult with a professional HVAC contractor who can assess your specific needs and recommend the appropriate materials and installation methods for your situation. They will consider factors such as local building codes, climate, and the specific requirements of your HVAC system to ensure a proper and reliable installation.