The biggest potential proble I see is that your alternator probably puts out more than 75 amps. That means if you completely drained both rear batteries, the alternator would basically put everything it had into charging them, potentially blowing both 75 amp fuses.
You also need to verify that the connector can handle the necessary continuous currrent.
A cursory search on the interweb indicated that H3's have 120 amp alternators.
Steve - '97 LX 450 with Slee 6", 35" Toyo's, Slee front, 4X4 Labs rear, Hanna sliders, 5.29's, front and rear chromoly axles and the two Dober's: Zeus and Zoey