I was wondering if Old Bruno's mechanic of "Play: Capture 3 æmber" means that your opponent must have at least 3 æmber in order to capture any since the mechanic does not strictly say "Play: Capture UP TO 3 æmber"?

In contrast to many other card games, this is not a “cost” or “requirement” to play. You simply “do as much as you can, and do all you can”, meaning don’t capture less than 3 if you can, but if the opponent has less Amber than 3 you capture all you can.

The "do as much as you can" thing feels like a placeholder for actual targeting rules.