You should have them separated, as by the time the 55 gallons cycles through (a few weeks at minimum) you will end up having many issues with just the 3 girls.. most likely one will be killed and a second one just about close to that by the end of a couple weeks. Highly recommend you separate the 3 girls until you have the appropriate number.
You can always just clean out the algae yourself so the oto can be placed in the group it needs.
As for introducing the girls.. once the 10 gallon is empty of all other fish, rearrange it and float each girl in their own cup and acclimate to both temp and chemistry as you would normally. It's always a risk when setting up sororities, as we are forcing them to be together when they naturally wouldn't be constantly together. Having only 3 right now, 2 will gang up on 1 and wear her down til she can't take it anymore, then the 2 will battle for the tank. The reasoning behind 4/5+ is that it allows the fighting and chasing to be spread out between enough of them as to hopefully avoid just one being bullied. Even then it's still a risk- weeks, months later a fight can easily break out and you will have to separate them all and keep them separated. One of the general rules is make sure you have enough spare tanks to home each girl permanently if the need arises, as it's a high risk enviroment. So would be recommended to separate them for their own safety and health for now. Or.. clean out the algae on the glass yourself, grab a couple of ghost or cherry shrimp to put in to clean the tank so you can get the other girls in there asap.
Breeder so I have too many to count and too many named to list here!