There are few downloads released for Microsoft SQL Server Compact 3.5. Here is quick lists of the same.
Microsoft SQL Server Compact 3.5 Service Pack 2 for Windows Desktop
SQL Server Compact 3.5 SP2 is an embedded database that allows developers to build robust applications for Windows desktops and mobile devices. The download contains the files for installing SQL Server Compact 3.5 SP2 and Synchronization Services for ADO.NET version 1.0 SP1 on Windows desktop.
Microsoft SQL Server Compact 3.5 Service Pack 2 Server Tools
SQL Server Compact 3.5 SP2 Server Tools Windows Installer (MSI) file installs replication components on the computer running the Internet Information Services (IIS) for synchronizing data with SQL Server 2005, SQL Server 2008 and SQL Server 2008 R2 November CTP.
Microsoft SQL Server Compact 3.5 Service Pack 2 Books Online
SQL Server Compact 3.5 is a small footprint in-process database engine that allows developers to build robust applications for Windows Desktops and Mobile Devices. This download contains the Books Online for the SP2 version of SQL Server Compact 3.5.
Note: The brief description below the download link is taken from respective download page.
Reference: Pinal Dave (https://blog.sqlauthority.com)