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.

If you're thinking about bringing a Raspberry Pi on a road trip, your first thought might be to bring along a RetroPie rig (here's how to install RetroPie on a Raspberry Pi 4 if you want to see ...