Will Zigbee 3.0 devices be backward compatible with older devices?

Yes, in some scenarios.
A Zigbee Home Automation device can join a Zigbee 3.0 Centralized Network that does not require a key exchange.
A Zigbee Home Automation device can NOT join a Zigbee 3.0 distributed network.
A Zigbee 3.0 device can join either a Zigbee Light Link or a Zigbee Home Automation network using classical commissioning.

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