how does astral heron work for a chain when playing your first card. what counts as the first card played in this example: You play temporal breach, they counter with repulse, you defy it, they play another repulse targeting breach. So repulse2 resolves, defy resolves, repulse1 gets countered, temporal breach gets countered. your first card was breach but it never resolved. but you played defy and it resolved. does that count as your first card played now? And if you played 3 reactions ontop and the last one resolved, does that mean you played your first card even though it was the 3rd card from hand but first to resolve?

Yes — "your first card played" is determined by which card finishes resolving first, not by chain order, hand order, or how many cards got countered along the way.

In your stack the items resolve newest-first: Repulse 2 → Defy → Repulse 1 → Temporal Breach. Defy sits above Breach, so Defy resolves first, and that's the first card you played this turn — Heron triggers then (as long as Heron is at a battlefield at that moment, per Rule 383.2.a.1) and your next card this turn costs [2][A][A] less. Breach never counts: a countered card isn't considered played at all (Rule 419.4.a.1 / 425.1.b), and it resolves last anyway. Same answer for your second question — if you stack 3 Reactions, the last one you played resolves first, so a card that was third out of your hand can absolutely be your first card played, while the two beneath it (if countered) never count.

Small correction on the example itself: Repulse only counters an enemy spell that chooses a single friendly unit at a battlefield, so it can't legally be aimed at a Defy (Defy chooses a spell, not a unit) — a legal chain there would have Defy countering Repulse 1 instead.

Two limits worth remembering: Heron triggers only once per turn and only off the first card that resolves, and the discount must be used this turn (it expires in the Expiration Step). Also note the split with Rule 419.4.b — a countered card still counts for static checks like Legion, just not for "when you play" triggers.