if my opponent plays dragon with ability 'kill a unit' and then i use temporal breach on my watcher. does the dragon get -3 or no. does the order of chain matter or is the unit played already and just its ability on chain is unresolved?

Yes — the dragon gets -3 Might, and chain order doesn't change that.

The dragon resolved as a permanent the moment it was played; only its "when you play me" ability is sitting unresolved on the Chain (Rules 359.2, 383.4.a.2). So when your Watcher's -3 resolves, the dragon is already an enemy unit on the board and gets -3 (to a minimum of 1) — same as any other unit present at that time.

One thing order does affect: Temporal Breach banishes your Watcher and replays it as a new game object. So if the dragon's kill/damage ability had already targeted the Watcher, it will mistarget and do nothing to the replayed Watcher (Rule 359.3.e.4). The -3 doesn't stop an outright "kill" or Elder Dragon's lethal 1-damage otherwise — but in this case the original target no longer exists.