Can I run a single Magento for FR + BE + LU?

Yes — Magento’s multi-store architecture handles FR + BE + LU cleanly because all three share French-language content, EUR currency, and similar (but not identical) tax rules:

• Single Magento backend, 3 stores — one for FR, one for BE (FR-language), one for LU. Shared catalogue, separate URLs (yourbrand.fr / yourbrand.be / yourbrand.lu) or one domain with country selector.
• Per-store TVA — FR 20%, BE 21%, LU 17%. Each with its own reduced rates. Magento tax_zone rules per store.
• Per-store payment methods — FR adds CB + PayPlug + Alma, BE adds Bancontact, LU keeps it lean. Configured per store.
• Per-store shipping — Mondial Relay for FR + BE, bpost for BE-only, POST Luxembourg for LU.
• Per-store legal pages — Mentions légales (FR), Conditions générales (BE), Conditions générales (LU). RGPD applies to all three but enforcement is via CNIL (FR), APD/GBA (BE), CNPD (LU).

For Belgian Dutch-speakers (Flanders), you’d add a 4th store with NL-language. Architecture decided in the audit step.