As LibertyBelle said, the answers are:
1= A or B, depending on context...
2= B
3= A or B, depending on context...
...and here is why:
"at" and "in" both show the place we are talking about.
"at" talks about the place and that area, the building and the area that the building is in, for example. "at" shows a wider area than "in", in your sentences.
As you are talking about buildings, "in" is better if you specifically want to say "inside" as opposed to "inside or maybe just in that area", for example:
[1] Where were you Friday night when I came to your house? I knocked the door but no-one answered.
[2] I was at home. I was probably in the garden.
The garden is NOT inside the house, but the garden is "part of the home" area.
In short, "at" usually shows a wider area than "in" if used with places like buildings, but of course we do say "I am in England rightnow", too, so it depends on context.
