I couldn't find the SmartGWT 3.0 entry for maven.
For 2.4, I found below link:
http://code.google.com/p/smartgwt/wiki/NightlyBuilds_and_MavenRepository
Have you tried: http://www.smartclient.com/maven2/com/smartgwt/smartgwt/3.0/smartgwt-3.0.pom ?
Repository: http://www.smartclient.com/maven2
<dependency>
<groupId>com.smartgwt</groupId>
<artifactId>smartgwt</artifactId>
<version>3.0</version>
</dependency>