The full address is:
Flat 3B, 3 Hans Crescent
from the embassy website http://www.ecuadorembassyuk.org.uk/contact
The “B” implies that the embassy occupies one of the floors, not the entirety of number 3. Which floor depends on how they are labelling them. The usual way in the UK would be for the ground floor to be plain “3”, then the next floor up “3A” etc. Now, it looks like this building has a mews level, so who knows, it may just be the one floor above ground level, rather than the second.
In any case, the front door that the police have been moving through is the door into the building that contains the embassy, not the door to the embassy itself. Any arrest, should Assange choose (heh) to leave, will surely take place inside the building, at the door of Flat 3B, and not on the street.