If I attack with Ezreal Dashing a battlefield occupied by a Scuttle, can opponent react with Rengar Trophy after Scuttle is killed by Ez effet? And does Ezreal effect trigger again ? Can he see my hand before deciding to react with Rengar? And if he reacts to the trigger "When I attack" with Rengar, can I choose him as a target, or I can only select Scuttle?

Yes on the Rengar window, no on a second trigger.

Rengar's errata'd text ("I can [Ambush] to a battlefield where there are enemy units, even if you don't have units there") lets him be played as a Reaction to that battlefield as long as your Ezreal is still there. So the opponent can respond either while the "When I attack" trigger is still on the chain, or after it resolves and Scuttle dies — the Showdown only closes once all players pass focus in sequence, so they always get a window (Focus stays with you after a trigger chain resolves, Rule 346.1, but you must pass it eventually).

Ezreal will not trigger again. Attack triggers are checked once per combat, when the unit first gains the Attacker designation (Rule 383.4.e.2.a). Rengar arriving mid-combat is just a new Defender in the same combat, not a new combat. (Only if a genuinely separate combat were later staged at that battlefield would Ezreal be the Defender there, and his ability would trigger off defending — once.)

Yes, he can see your hand first. Scuttle Crab's Deathknell is mandatory, so it goes on the chain the moment Scuttle is killed and resolves before the Showdown can close — you're the chosen opponent, so you reveal your hand and the Scuttle's controller may look at your facedown cards this turn. Nothing about that resolution depends on him committing Rengar, so he gets the information before deciding.

Last part: only Scuttle. Targets for a triggered ability are locked in at finalization on the chain (Rule 352.4.b), which happened before Rengar existed at that battlefield, so you cannot point the damage at him — the 3 damage resolves into Scuttle.