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