You might just cage or put the BOs in the bathtub (adding something "Non_slippery" to it first... pine shavings and hay over cloth or cardboard?). Just while your gone, or just until the next morning when you can be there to monitor their integration. And, they will need water and food, water that is not deep enough for them to get into and drown in.
Three week old chicks will pick on younger chicks, regardless of breed. "Pecking order" and over food.
Later.... when they are all together in a bigger set up, brooder, coop..... You can get a bale of Alfalfa hay.. it's costly and getting more so but, they like it, it's good for them, they will "scratch" in it, on it, at it.. . and it will keep them busy (any age chicken). You can have more than one food dish and more than one water dish.. and you should watch for picking and wounds from picking when they huddle together and/or sleep together (everyday, every am). Cabbage (as well as Alfalfa hay) works for "busy work" for the flock when chicks are introduced, it will keep the older ones busy and distracted, for a little while.
Other than that, maybe a cut in half zuchinni squash... there isn't much that very young chicks can eat, or should eat, besides chick starter. (I am mentioning the Alfalfa hay and cut in half or peeled zuchinni as "busy work" to help keep them from picking at each other. Two feed dishes and two waterers when you have room will help them not to fight as much to eat. You can provide them with grit (w/oyster shell in it)... and, a sand box if the are in the coop and run and not confined when you have room.
Sounds like your biggest concern is just not being home one evening?
Do you have a coop, run, flock that these will all ultimately be added to? A cage set within the coop, flock area with all they need in it, and the flock's food and water just up against the outside wall of the cage will help everyone to "get aquainted" for a day or three by "eating and drinking together", to be head to head.
Remember, little chicks have to be inside or have warmth for five weeks or until they have feathers on their heads and all over... are completely feathered out.
Do you have an extra bathroom and tub? Needs to be made "non-slippery"... but, my large tub has been a brooder several times.
Even a cardboard box will do for a night, but you will need food and water that is safe for little chicks, that they won't fall into and not be able to get out.. chicks have drowned in puddles from rain, water dishes too deep to get out of.
Just some thoughts... some ideas... depending on your situation and set up..... Best of luck.
