When Lightning Rush looks at the top 3 cards and I see Undertitan, does that add 2 energy?

No. Lightning Rush says "Look at the top 3 cards," and looking is not revealing — Undertitan's energy ability only fires when an effect explicitly uses the word "reveal" (RiftJudge FAQ #4282/#251).

Same as Hook and Baited Hook: you can choose and draw the Undertitan, but you get no [2] Energy from it. Only effects that literally say "reveal" (like Rift Herald's move trigger) will set it off.