You would use this node syntax:
AIP-3.9.2
The syntax is SEGMENT - FIELD [OPTIONAL SUBFIELD INSTANCE] . FIELD . FIELD
To get the Subcomponent1, you would use this message.getNode():
message.getNode('AIP-3.3.2')
To replace Subcomponent2, you would use this message.setNode():
message.setNode('AIP-3.3.3')
Subcomponent1 and Subcomponent2 are part of AIP-3.3; using message.getNode('AIP-3.3') will return Field3&Subcomponent1&Subcomponent2.