ICSD is probably the best choice for finding crystal structure information for inorganic substances. It actually contains the .cif files for the substances, and you can download these files for use in any visualization or statistics program that will read them. Unfortunately, it is also the most complicated of the three options for finding crystal structure information.
Reaxys does not contain actual .cif files, but it does has some crystal structure information present in its database, as well as referring you to articles in the primary literature that contain such data. Searching for crystal structure information in Reaxys can be challenging since there are several fields that could potentially contain it, and some of the more interesting fields, such as the dimensions of the unit cells, are browsable but not searchable. However, it is a good second choice if ICSD does not contain the information you want.
Click here for more information on finding crystal data in Reaxys.
The staff at Chemical Abstracts Service has been working overtime to add property data to the SciFinder substance records; however, most of the crystal structure data consists of references to the primary literature in which they were published. There are no .cif files in SciFinder, and, in many cases, the property values are not presented in the database itself. However, since every paper in Chemical Abstracts is indexed with the roles of all important substances that appear in it, SciFinder can help you find articles that include the crystal structure of your choice.
Click here to learn to use SciFinder to find articles that reference crystal structures.