This issue was originally reported on the forums:

https://discuss.gradle.org/t/war-plugin-configuration-lifecycle-regression-in-gradle-3-0/18985

This looks to be a side effect from the Groovy to Java conversions. What was done is that the War task copy specs are being build from Iterable<File> rather than a FileCollection. The result is that when we traverse the task dependency graph we trigger dependency resolution in order to retrieve the Iterable<File> rather than relying on the laziness of FileCollection.

https://github.com/gradle/gradle/blob/REL_3.0/subprojects/plugins/src/main/groovy/org/gradle/api/tasks/bundling/War.java#L70-L73