How and where to declare common dependencies between modules

let say module A, module B, and module C all use the testNG library do we need to add this dependency in three build.gradle files or is there a better solution.I have kept dependencies in build.gradle of root, but get the error no methods found for testImplementation

subprojects{
apply plugin:'java'
    repositories {
        mavenCentral()
    }

    dependencies {
        testImplementation group: 'org.testng', name: 'testng', version: '6.14.3'

    }
}