Ensure you downloaded the rpi3 build, not rpi4 or rpi2. Also, check your power supply. The Pi 3 needs a 5V/2.5A micro USB power supply. A phone charger (5V/1A) will cause boot loops.
: Insert the card into your Pi and power it on. Batocera will automatically resize the partitions to fill the entire 32GB of space on the first boot. download batocera 32gb raspberry pi 3 link