It's all in the wording:

 

The minimum visibility required by JAR OPS 1 to commence a special VFR flight is: 3km

 

To conduct a special VFR flight, the minimum visibility required by JAR-OPS 1 is: 1.500m

 

i.e. you cannot take off with a special VFR clearance unless the visibility is 3km or above. However, once in the air, you can fly on a special VFR clearance until the visibility drop below 1.5km