If I move a rift herald to a battlefield hold by the opponent, with a hidden hidden blade, and in respond to my move trigger, the hidden blade is played against my rift herald, do I still get to look at the top 3 cards with the move trigger effect?

Yes — you still look at the top 3. Rift Herald's move trigger goes on the chain the moment it moves, and once a triggered ability is finalized to the chain it resolves independently of its source (Rule 383.3). Nothing in "look at the top 3 cards of your Main Deck" references the Herald itself, its location, or its Might, so killing it with Hidden Blade can't make the trigger fizzle.

Order of resolution: Hidden Blade finishes killing the Herald → you draw 2 (the kill instruction succeeded, so the linked draw executes) → the Herald's Deathknell is a pending item placed on top of the chain, so it resolves next → then the move trigger resolves, letting you look at the top 3, reveal a unit and draw it, and recycle the rest.

Note this is different from abilities that use "here" or "my Might" — those check the source at execution and would come back null if the source is gone; Rift Herald's move ability does no such check.