@lilejustin i expect your old pi3 was a model b. do you mean you now have the pi 3 model b+? that could require a firmware upgrade that your old image may not have had. i suggest you start again: https://retropie.org.uk/docs/First-Installation/
you could try and harvest the old games and settings from your old sd card via an sd card reader and an OS that can read the ext4 partition, but IMO this is ultimately more trouble than just starting over.