If I conquer with Kennen Storm of Shuriken, can I play Drag Under from trash for 3 energy 1 power?

Yes, you can play it for 3 Energy and 1 Power.

Here's why: Kennen gives Drag Under Flow "equal to its cost." Per Rule 131.4, "its cost" means the printed cost of 5 Energy and 1 Power — so the Flow cost is set to 5E/1P.

But when you actually go to play it from trash, Drag Under's own ability triggers a discount during the cost-determination process: "I cost [2] less to play from anywhere other than your hand." Since trash is not your hand, this reduces the Energy cost by 2 (Rule 356.4). The final cost becomes 3 Energy and 1 Power.

This is different from a cost check like Kai'Sa's eligibility filter (FAQ #11602), which uses printed cost. Kennen sets the Flow cost, and then Drag Under's own discount applies during play as normal.