How does repeat work with Lux legend? For a base 3 cost and a base 5 cost spell?

Base 3 cost spell: Lux does not trigger. Lux's legend ability checks the printed cost (the number in the top-left corner), not what you actually pay. A base 3 spell is below the threshold, and Repeat is an additional cost that doesn't change the printed cost (Rule 206; FAQ #10574).

Base 5 cost spell: Lux does trigger. The printed cost is 5 or more, so her legend ability fires regardless of whether you pay Repeat. Repeat adds extra cost but doesn't change the printed cost — the spell still "costs 5 or more" for trigger purposes. She'll draw you 1 card after the spell resolves.