How are Zener maximum power ratings derived?
I've sampled a few rows of the table for Zeners of different voltages. It seems to be consistent that the product of the nominal voltage, and the \$I_{ZM}\$ is about 1.5W, half of the 3W.

Now, that 3W figure is something that has to be derated for ambient temperature. So there are conditions at which the maximum dissipation will in fact just be 1.5W.  The derating is 24mW above 75 Celsius, so if the ambient temperature is 137 degrees, it is down to 1.5W.

So, the maximum current values can be regarded as simply being conservative. You can have that much maximum current even if the ambient temperature is quite high. Note that the table does not say anywhere that you must derate \$I_{ZM}\$ for temperature!

It looks like to achieve close to the 3W dissipation, the ambient temperature has to be 75 or less (since the 3W derates above that), and you have to violate \$I_{ZM}\$ (since those values are quoted for 1.5W dissipation).