Can Fizz play Drag Under from the trash since its cost becomes 3 mana and 1 Power? When Fizz checks for its costs, does he see Drag Under as a 3 cost spell or does he see it for its original cost of 5?

No. Fizz, Trickster cannot play Drag Under from the trash.

Fizz's ability checks the spell's printed Energy cost (the number on the card), not any discounts that would apply while playing it. Drag Under costs 5 Energy printed — well above Fizz's limit of 3. Even though it would cost only 3 when played "from anywhere other than your hand," Fizz's check happens before any cost modifications, so it sees 5 and can't target it.

Rule 131.4 confirms: effects that determine a card's cost for any purpose always use its printed cost, even if that cost is altered or ignored later. FAQ #7430 directly rules on this exact scenario: Fizz Trickster does NOT work with Drag Under.