Looking through the product brief and feature summary list (hardware features), there are only 2 differences, connectx3 supports energy efficient ethernet and connectx2 is compatible with 10Gb no mention of 40Gb like the x3. EDIT: forgot to add Precision clock sync is also in x3. The 2 documents are almost identical, a few things moved about and a few things removed due to duplication in the feature list. So as Lars has said, there doesn't appear to be much difference between x2 and x3 apart from speed, also the product briefs say for feature lists, refer to the driver documentation.
"*This product brief describes all of the hardware features and capabilities. Please refer to the driver release notes on www.mellanox.com for feature availability."
So as the hardware feature list is almost identical, all the additional features are implemented in driver.
EDIT: Also the "silicon" documents are here:
http://www.mellanox.com/related-docs/prod_silicon/PB_ConnectX3_VPI_Silicon.pdf
http://www.mellanox.com/related-docs/prod_silicon/ConnectX-2_Silicon.pdf