Are DOCSIS 3.0 modems backward compatible with DOCSIS 2 ?

Yes, DOCSIS 3.x hardware if fully backward compatible with previous DOCSIS versions.

In general DOCSIS 3 can give you a more stable connection even when connecting to a DOCSIS 1|2 plant, however, you won't get higher speeds.

DOCSIS 3 modems allow for better, more stable performance and support channel bonding
DOCSIS 2 modems introduced more available bandwidth
DOCSIS 1.1 modems introduced QoS features to the MAC layer

D3 modems have better hardware, faster CPUs and their higher capabilities may perform slightly betted than D2 modems even when connected to an older DOCSIS 2|1 plant.