Sorry about the delayed reply!
Apart from probably needing to update Spring if you are using it in your project, the DWR 3.0.0 release notes (see http://dwr.2114559.n2.nabble.com/Announcing-the-official-release-of-DWR-3-0-0-td7581090.html) should be a good indication of what you need to look at:
DWR 3.0 RELEASE NOTES (issues resolved since 3.0 RC3)
=====================================================
Potentially Breaking Changes
----------------------------
[DWR-645] - Clean up path configuration options
[DWR-642] - Remove legacy packages
[DWR-639] - Simplify destroy event methods
[DWR-511] - Evaluate ServerLoadMonitors, remove unneeded
monitors, solidify monitors we want to keep
Security fixes
--------------
[DWR-638] - Improve DWRSESSIONID cryptographic strength
[DWR-632] - Improve CSRF protection to handle more scenarios
New Features and Improvements
-----------------------------
[DWR-648] - Add support for Cache-Control and Expires header
handling
[DWR-647] - Allow customization of DWRSESSIONID cookie
[DWR-644] - Provide a standard way to specify client-side options
before DWR loads (dwrConfig)
[DWR-643] - Allow remapping of DWR's effective contextPath (for
DWRSESSIONID cookie etc)
[DWR-641] - Offer more access to servlet request from Reverse Ajax
[DWR-636] - Rename allowGetForSafariButMakeForgeryEasier setting
[DWR-628] - Add support for Spring Java configurations
[DWR-587] - add support to > ("Content-Disposition", "inline;
filename=")
[DWR-547] - Update class-mapping documentation to mention
automatically generated inheritance scripts
[DWR-110] - add name (javascript) attribute to DataTransferObject
annotation
Bugfixes
--------
[DWR-649] - Optimizing proxies are removing meta-data in DWR
responses
[DWR-646] - Incoming request attributes should be available to
ScriptSessionListener.sessionCreated()
[DWR-640] - Reverse Ajax polling mode broken
[DWR-635] - New callbackArg option causing conflict when delegating
functions
[DWR-634] - Auto-filled servlet objects (request, session, etc) in
method parameters not matched correctly
[DWR-631] - FileStoreDownloadManager - encodeFileNameSegment
[DWR-629] - DefaultCreatorManager throws NullPointerException
during destroy of Spring context
[DWR-627] - IllegalStateException on Jetty 8 and 9
[DWR-614] - org.springframework.util.ClassUtils.forName(className)
has been removed in Spring 4.x
[DWR-585] - java.io.NotSerializableException:
org.directwebremoting.impl.DefaultContainer
[DWR-554] - Reverse ajax client side timeout is missing
[DWR-396] - Transfering a file using DWR 3.0 disables the "input"
events
[DWR-378] - Window.dwr access permission denied
[DWR-352] - Javascript error in engine.js when file input field
has no parentNode