Like all +3 weapons, the DM will add +3 to the AC hit when using this weapon but the damage bonus has already been factored into the damage wheel.
Welfor's +5 Slayer Sword
Adds +2 to Strength (thus adding +1 To Hit in melee and +1 Damage with melee and thrown weapons)
+3 to Strength (thus, depending on prior Strength score, adding either +1/+2 To Hit in melee and +1/+2 Damage with melee and thrown weapons) and +3 to Dexterity (thus, depending on prior Dexterity score, adding either +1/+2 To Hit with ranged attacks, +1/+2 to AC, and +1/+2 to Reflex saving throws)