Is DDR3 memory backward compatible with DDR2 and DDR?

Answer

No. DDR3 is not backward compatible with DDR2 and DDR. e.g. DDR3 has 1.5V power, DDR2 needs 1.8V and DDR needs 2.5V power. In terms of standard motherboard slots also the notch or the key for different memory modules is at different place for the three type of memory thus preventing any accidental insertion in memory slots which can damage the memory and or the motherboard.

More Information:
Gateway Tech Support said their Gateway MD7820u Notebook is compatible with both.
SODIMM DDR3 might have the same pins as SODIMM DDR2.