You could keep them in the same tank until you can determine gender. If you discover that two or more of your babies are males, separate them in different tanks. Multiple females can be housed together as long as they have enough places to hide and get out of each other's sight. Most babies are incubated for females so chances are you'll get mainly females.
Also, keeping 3 babies in such a large tank may produce a problem. If you're feeding with crickets, the more space and the more hiding places the crickets have, the harder it will be for the babies to find and eat them. I suggest having a small bowl of mealworms available at all times just in case the babies can't find the crickets in the cage.
An important fact to remember is that crickets decrease in nutritive value very quickly if they roam too long in the cage. You should only feed as much as the geckos can eat. Excess crickets can also stress out the babies.
I suggest going
HERE to read up on leopard gecko care. Garrick and Sheila DeMeyer of CrestedGecko.com wrote it and they're one of the more prominent breeders out there. I've ordered geckos from them in the past and they sure know what they're doing.