Extension cables

I have already discovered several extension cables with flat plugs. They are cheap because you can also plug them in behind cupboards. However, the plugs of the cables I have found so far are designed in such a way that they partially cover the two other sockets in triple sockets. The result: you can only use one of the three sockets. Does this have to be the case? Or is there a three-metre-long extension cable whose plug is not only flat but also designed in such a way that the two other slots remain free?