JMO: you can't force dogs to love each other. You can strictly enforce behavior and hope they continue to tolerate one another, but that's the limit of what you can do as a human.
If the 8 mo old is behaving appropriately- I'm talking good manners in 'doglish' here, not what WE think of as appropriate as humans- I'd probably put the 3yo on a 'nothing in life is free' program and make sure she clearly understands YOU are running the show. That way if/when bickering starts you'll have a better chance of stopping it before things get out of control. Controlling toys and food closely is a good idea too.
FWIW if any of mine get in a cussing match, all participants get cuffed and shoved into an immediate down next to each other. Then I quite dramatically ignore them while showering attention on the GOOD dogs, who are still running around freely... and God as my witness, the innocent doggie bystanders seem to enjoy rubbing it in when someone else gets in trouble! If the guilty parties screw around or try to get up, they're going to get another lecture and dumped right back into a down like a lowly sack of potatoes.
By the time all this drama wraps up and I let them go, the combatants usually trot off together looking over their shoulder at me like two little boys spending the rest of the day commisserating after getting busted down for arguing over a ball: "holy smokes, isn't she MEEEEEEN!?!" "Yeah! MEEEEEEN!"
Note that this is for minor squabbling, not flat-out "I'm going to kill you" fighting.