Hi there,
I went back to work 3 days a week too after having my DS, and we had a nanny.
I introduced him to the nanny at her interview, and then she did a 3 day week as her first week, with me, the week before I went back to work.
The first day, she was with the both of us all the time. I showed her where things were, what DS liked etc. The second day, I left her alone with him for an hour or two in the afternoon whilst i went out 9to the supermarket, I think!). And the next day, I went out for the whole afternoon (shopping, to buy new clothes for work).
And then the next week she started her 3 days a week on her own with him. It worked really well.
On the practical side, whilst she was doing ehr week whiilst i was there, I drove her round showing where the local places were eg where DS went for his classes,the doctors, parks, local hospital, where friends lived etc. I also did a list of emergency contact numbers, got a set of keys cut for her etc.