Just wondering if there's any docking solution that provides the possibility to connect to 3 external displays to the MacBook Air 2018, or if we're hardware limited by the iGPU here.
If you want a "native" display signal, the 2018 Retina MacBook Air is limited to 2 external monitors.
However, you can use other types of displays such as a USB display to act as a third monitor. They are generally limited in resolution, update frequency, GPU performance, etc. - so do not expect to be able to play 3D games or use 4k resolution on such a display.
If you want higher performance for the third display, you can connect an eGPU to the MacBook Air which would enable you to use more monitors with higher performance. Such a setup is usually quite more costly than a USB display however.