'18 Mustang, Sync 3.0 - Can I update maps with Cyanlabs updater, without updating Sync?

No, you can’t: Syn3updater only works with a single 3.3 and 3.4s, and maps are not 3.0 compatible…
You could install maps on 3.0 for NA but you will require a license…
You can take this guide as reference → Tutorial: Sync 3.0.20204 with 2020 NA2.20 Map Installation