Thermal Runaway in Diodes
You are correct about reduction of \$V_f\$ with increasing junction temperature. But, this isn't about forward bias because lower drop in a single diode is not a problem. The diode doesn't determine the current flow. Instead it's about leakage under reverse bias. The datasheet appears to make the claim that under application of 3A forward current at 75C ambient, no significant hot spots will develop. Hot spots can cause failure under reverse bias due to increased leakage. 

With increasing temperature, diode leakage will also increase. A hot spot can create a local region with lower resistivity and leakage, which increases loss contribution from reverse bias. A hot spot will cause current crowding, and the device can fail due to increased current in the region with lower resistivity. This is thermal runaway for a single diode.

This is usually more of a problem with high voltage parts and Schottkys since they have higher leakage to start with. 

For more than you might want to know about thermal runaway, you might look at this article. 

Finally, no, you would not want to parallel these because they will not share current.