Those are the attack bonuses for iterative attacks in a full attack action for this creature with that weapon. This creature gets three attacks in a full attack, at +20, +20, and +15, each of which deal out
3d6+27+1d6 acid damage. It includes whatever buffs you have activated in Hero Lab - so if you have Haste on in the Adjust tab, then there's an extra attack for Haste in that line - or not, if not.
Here's the witch I was playing today's weapon tab, showing the normal 2 attacks for a BAB above +5:
And then again with Haste applied:
This is just like in a creature's stat block. In fact, you could output the stat block of this creature in Hero Lab if you're unclear how things are mapping to each other (File... Output Hero Statblock...).
Sredni Vashtar's Girl
Female Human (Vudrani) Witch 12
NG Medium humanoid (human)
Init +9; Senses Perception +7
AC 24, touch 15, flat-footed 21 (+4 armor, +4 shield, +3 Dex, +1 natural, +2 deflection)
hp 108 (12d6+51)
Fort +11, Ref +13, Will +14
Speed 30 ft.
Melee +1 mithral dagger +8/+3 (1d4+2/19-20)