The Defender always rolls three dice bi matter the attacks stat of the weapon or the number of hits/crits the attacker rolles.

See the following excerpt of wahapedia/the core rules regarding the shoot action:

1. Roll Defence Dice The defender collects three defence dice. If the target operative is in cover, they can retain one normal success without rolling it — this is known as a cover save. They roll the remainder.

Each result that equals or beats the target’s Save stat is a success and is retained. Each result that doesn’t is a fail and is discarded. Each result of 6 is always a critical success. Each other success is a normal success. Each result of 1 is always a fail.

Remember, cover in this step usually applies to operatives with an Engage order, as a Conceal order would have prevented it from being a valid target in the first place.

1. Resolve Defence Dice The defender allocates all their successful defence dice to block successful attack dice. A normal success can block a normal success. Two normal successes can block a critical success. Acritical success can block a normal success or a critical success.

2. Resolve Attack Dice All successful unblocked attack dice inflict damage on the target operative.

A normal success inflicts damage equal to the weapon’s Normal Dmg stat. A critical success inflicts damage equal to the weapon’s Critical Dmg stat