Just going from memory, the card reader is physically slightly longer than a comparable 3.5" hard drive. When I put one in my A500 sidecar I had to remove the top cover and bezel. In an A3000 I think it would fit without those modifications as long as it's not physically too long, although you might still need to do a little trimming around the edges of the bezel.
For what it's worth they fit perfectly in an A2000 with no modifications. Here's my favorite picture of mine.
