The Acer Predator Orion 3000 is a gaming PC that can be configured with various components, including different CPUs and GPUs. The frame rate (FPS) you'll achieve in Fortnite on all low settings depends largely on the specific configuration you have, particularly the graphics card and processor.
For a standard configuration with a mid-range graphics card (like an NVIDIA GTX 1650 or GTX 1660), you can expect to achieve around 60-120 FPS on low settings at 1080p resolution. If you're using a higher-end GPU (like the RTX 2060 or RTX 3060), you might see FPS in the range of 120-240 FPS or even higher, depending on other system specs and background processes.
To get the most accurate estimate, it would be best to look at the specific GPU and CPU in your Orion 3000 configuration. If you provide those details, I can help you more precisely!