Is Keeper of the Light in the trash but her condition is fulfilled, does she count as a 3 cost or a 5 cost for the purpose of summoning rules on other cards?

It counts as a 5-cost (printed cost), not 3.

The card in the database matching your description is Keeper of Law — printed cost [5] (plus 1 Power). Her "I cost [2][C] less if…" discount only applies during the play process when she's actually being played; it never changes her cost for other effects.

Per Rule 206, any effect that needs to determine a card's cost "for any purpose" uses the printed or copied cost, even if that cost would be reduced, increased, or ignored as the card is played. So cards checking her cost while she's in the trash (e.g., "play a unit from trash that costs X or less" type effects) see 5, not 3 — the condition being fulfilled is irrelevant to them. This matches RiftJudge FAQ #5447 (Kai'Sa checks printed cost, not conditional reduced cost).

The discount would still apply normally if she's actually played while the condition is true — it just doesn't affect how other cards evaluate her.