It will be -3 in total. -2 for being in hard cover and -1 for having the Skirmishers special rule.

During the Shooting phase, your army lets fly with the missile weapons at its disposal, be they bows, cannons or fantastical war machines. Most armies will have a unit or two of archers, crossbowmen or mighty war machines, and use these missile units to thin the enemy ranks before the bloody press of melee begins in later turns. Other armies will field masses and masses of missile units, intending to seize victory in the Shooting phase by obliterating the enemy at long range. This chapter covers the shooting rules for common weapons and the majority of troop types. War machines and other, more peculiar, devices of destruction are covered in their own chapter. However, even the most colossal trebuchet is governed by many of the same rules as the humble bow and arrow, so it’s worth reading through this chapter before unlimbering your cannon or organ gun.

Shooting

As per the 'Resolving Unusual Attacks' box. Note that all hits resolved in this way will count as shooting attacks for working out who is hit.

No they count as an unusual attack and will be distributed as a shooting attack.

Warhammer Fantasy: 8th Edition

Shooting - Frequently Asked Questions
URL Copied!

What will the To Hit modifier be for shooting at a unit in a building with the Skirmishers special rule? (p40)
URL Copied!

When hits, other than from shooting and close combat attacks, are inflicted upon a unit how are these hits resolved? (p42)
URL Copied!

Do special rules that can inflict hits in close combat, such as Stomp and Breath Weapons, count as close combat attacks? (p42)
URL Copied!

When a unit has multiple toughness values or armour saves do you use the value of the majority or in the case of a tie, the best? (p42, 43)
URL Copied!

A war machine is wounded by a hit with the Multiple Wounds rule. Is the number of Wounds lost limited to 1? (p45)
URL Copied!

Shooting - Frequently Asked QuestionsURL Copied!

What will the To Hit modifier be for shooting at a unit in a building with the Skirmishers special rule? (p40)URL Copied!

When hits, other than from shooting and close combat attacks, are inflicted upon a unit how are these hits resolved? (p42)URL Copied!

Do special rules that can inflict hits in close combat, such as Stomp and Breath Weapons, count as close combat attacks? (p42)URL Copied!