Do they? It's a percentage, it might not be additive and just sets the exp/job multiplier to 300% instead of applying it on the base percentage
Just quickly confirm, three rounds certatintly makes me doubtful since even basic jobs can't be leveld that quickly
Let's see...
After one bout of the three enemies, Hild with a Hard Job Wrist went from Nurse level 1 to Nurse level 9 with 101 exp left to level 10.
Black Tanuki went from Card Battler level 1 to Card Battler level 8 with 452 exp left.
-Used the job exp seed to reach level 10 for Hild.
After second bout, Hild with a Hard Job Wrist went from Doctor level 1 to Doctor level 8 with 250 exp left to level 9.
Black Tanuki reached level 10 in Card Battler.
After third bout, Hild reaches level 10 Doctor.
Black Tanuki with Hard Job Wrist went from Card Summoner level 1 to Card Summoner level 6 with 272 exp left to level 7.
Well, since I started the stage three job only with the third bout, I'll add two more.
After fourth bout, Hild goes from Godly Doctor level 1 to godly doctor level 6 with 434 left to level 7
Black Tanuki went from Card Summoner level 6 to Card Summoner level 8 with 880 exp to level 9.
After fifth bout, Hild with no Hard Wrist Job went from Godly Doctor level 6 to Godly Doctor level 8 with 1057 exp left to level 9.
Black Tanuki with Hard Wrist Job went from Card Summoner level 8 to Card Summoner level 9 with 703 exp left to max level.
Ok, three bouts is just short of maxing out an Advanced Job, but given that you can complete the Unlimited Class in just over 30 seconds by holding A and mashing if you have a good loadout and have the singers go first, me being off on the number of bouts is negligible.
If I had to guess... they are multiplicative, not additive. So a unit with increased exp gain due to either item or ability multiplies with the 200% and the 300% of the two songs, reaching x1200% exp.
Of course, the song exp boost only applies to the units fighting.
My math might be wrong, but if you can tell me how much a level 9 advanced job needs to get to level 10 from scratch, I can tell you exactly how much the exp is multiplied by exactly.