If I reveal Undertitan with Rift Herald movement trigger, do I still get the +2 energy?

Yes — as long as Undertitan is the card you actually reveal and draw. Looking at the top 3 does nothing on its own, but Rift Herald's trigger then instructs "you may reveal a unit from among them and draw it," and that is a genuine reveal from your Main Deck, so Undertitan's "As I'm revealed from your deck, [Add] [2]" replacement effect applies.

The energy is added to your Rune Pool at the moment you reveal it (Rules 370.1.b.1, 369), during the Herald's trigger resolution — it just sits there and is spendable for the rest of the turn until your next Main Phase wipes the pool (Rule 167). The trigger doesn't ask you to pay anything, so this is pure gain.

One correction to watch: if you look at the three and choose a different unit to reveal, Undertitan staying facedown among them was only looked at, never revealed (Rule 425.1/425.2.b), so no energy — the same reason look-based effects like Rek'Sai's deck digging give nothing unless the card says "reveal" (FAQ #1708).