Depending on how the game lingo works, I can think of several things that could mean:
Overheal, raising your current HP/MP to three times their typical maximum.
Certain foes can apply "reduced recovery" debuffs, and the 300% effectively negates that (1/3 recovery effectiveness debuff * 300% recovery item = 100% recovery)
Three members of the party fully heal
I assume we'll see these in play later; either causing a hitpoint/mana bar overflow, resurrecting "dead" party members, negating an "unwinnable" sequence, or just becoming functionally unkillable.