If a combat starts and there are 3 attacking units, and I use Counterstrike on my defending unit, does it block all damage from combat?

Yes, Counter Strike will block all combat damage dealt to your chosen unit during that combat.

According to RiftJudge FAQ #8527 and Rule 460.2.c.1.a, combat damage is dealt simultaneously to all units involved in the combat. Because Counter Strike creates a delayed replacement effect that prevents the "next time" that unit would be dealt damage, it applies to the entire simultaneous damage event.

Here is the breakdown of how this works:

1. Simultaneous Damage: Even though there are 3 attacking units, all combat damage is dealt at the same time during the Combat Damage Step (Rule 460.2.c.1.a).
2. Replacement Effect: Counter Strike acts as a replacement effect. When the game attempts to deal the total sum of combat damage to your unit, the Counter Strike effect replaces that event with an event where the damage is prevented (Rule 437.2).
3. Full Prevention: Because all damage from all attackers is dealt in that single simultaneous event, Counter Strike prevents all of it, regardless of how many units are attacking your chosen unit (RiftJudge FAQ #8527).

Summary: When you play Counter Strike targeting your unit, it will prevent all combat damage that unit would take during the upcoming Combat Damage Step.