You can't put any babies together after they've been separated. If you were to get three baby females straight from the breeder and they were never ever separated, it could work but you would still need a HEAVILY planted tank and at least larger than 10 gallons. 15L's is only good for one fish. Since these babies have been separated, they will now act like any adult betta; very aggressive to others and possibly could kill as well.
Also if you wanted a sorority (all female tank) you need at least 5 females in 10 gallons or larger, nothing less than that.
I agree it is very unlikely to work out. At the age we buy the babies in the store, they are super small but still very close to adulthood. You'd basically have to treat them just like an adult sorority because even if they show no aggression right away, they will start to very soon.
Did you just recently buy your tank? As it is too small for 3 bettas, I'd recommend getting at least a 10 gallon to divide for the 3 babies. You could get them all separate tanks too, but I think divided ones are much easier when it comes to water changes.