If I conquer with Kennan and can give a spell flow, can I give drag under flow with cost 3 energy and 1 power or is it printed cost of 5 energy 1 power?

No — it checks the printed cost of 5 Energy.

Kai'Sa, Evolutionary's trigger says "with Energy cost less than your points" — and per Rule 131.4, effects that determine a card's cost for any purpose always use its printed cost, even if that cost would be reduced when played. Drag Under's printed Energy cost is 5, not 3. The self-discount ("I cost [2] less from anywhere other than your hand") only applies during the playing process, after the eligibility check is done.

So unless you have 6 or more points, Drag Under cannot be chosen by Kai'Sa's conquer trigger. This is the same logic as Fizz, Trickster not working with Drag Under (FAQ #7430), and Kai'Sa not being able to play Sky Splitter based on its reduced cost (FAQ #3520).

If you do have 6+ points and play it via Kai'Sa, you ignore the 5 Energy cost but still pay the 1 Power cost (per the parenthetical on Kai'Sa's card).