Am I able to attach scavenged weaponry to BT-1 and add 1 damage to each of his attacks when using missile salvo? The special activation says "perform 3 attacks" while the scavenged weaponry says "when you declare an attack."

No. Each of the 3 attacks are separate attacks that are declared one-by-one.

In other words: you can exhaust Scavenged Weaponry for one of those attacks to gain +1dmg.

When you perform an attack, the first step is declare target, and declaring attack is the same trigger as declaring target.