This is why the RetroPie download page only has ISOs available for models as recent as the Raspberry Pi 4. However, as "abj" alluded to, that's not the only way to install RetroPie on a Raspberry Pi.

This is a Raspberry Pi 3 with 4 GB of eMMC stuffed onto a board that fits into an SODIMM socket. The pins on this device give you access to the GPIOs and the DSI connector. All you really need to ...