This answer has been completely rewritten.
TL;DR: Additional bounces do a flat 3, 1, and 1 point of damage.
I have tested Mutalisks extensively in the Harvest of Screams mission against frozen Probes and Nexus during Flash Freezes, comparing my findings to HOTS Unit Tester Arcade map.
Flash Freezes suppress Protoss shield regeneration, and I've made sure to only attack targets with zero armor and shield armor.
Firstly, it turns out Campaign Mutalisks do not do familiar Multiplayer damage of 9/3/1 with upgrade bonus of +1/+0.333/+0.111. Instead, they do a much nicer 9/5/3, though upgrade bonus is the same as in multiplayer. Hence, +3 Mutalisks do 12/6/3.333 damage over three bounces.
Secondly, Vicious Glave upgrade does not provide any extra damage to first three hits, despite its description. Three extra hits do /3/1/1 damage regardless of upgrades, making Vicious Glave a debatable choice even against Rapid Regeneration.
Table 1: Health of targets (40-HP Probes, first target sometimes a Nexus, or extrapolated).
Table 2: Damage per hit, row differences from Table 1.
Fractional health of units is rounded down for display, so any fractional damage seems to take away a whole hit point, until it evens out on third or ninth or whichever hit. This is seen in Table 2, which confirms the assumption that second hit gains +0.333 per attack upgrade, third hit gains +0.111 per attack upgrade, and consequent hits do not gain any damage.
Reference: a very thorough article on damage calculation in Starcraft II.