C.M.
If you are just wanting what is cost effective, then having someone in part time to watch both children would probably be the best solution. You didn't say how much it is costing you to send your daughter across the street, but if you are going to need someone to watch your infant for 12 hours a week, it seems like they would be able to watch your 3 y.o. as well for an additional fee. I guess it would depend on how much you want to pay a nanny vs. how much it costs to send the 3 y.o. out.
I'm not sure what it is you do part time, but it might just be more cost effective to stop. Or work at night when your husband comes home.
My kids are 18 mo. apart, so I understand that you feel "stretched". I honestly can't remember the last time I had a full night sleep, a real nap, or time to myself.
Please remember that your 3 y.o. is being 3. She will regress with the potty, not only because of a new brother, but because that's what a lot of 3 y.o. do. And I wouldn't worry about stimulating time spent with her. Just do the best you can. She will be fine. I remember feeling all crazed about spending time with my kids but really, they only remember things that were fun. Not the times that I sat down in front of a cartoon with my 2 y.o. and infant. And dozed.
Good luck!