There doesn't seem to be a clear consensus about who is responsible for sending texts from 3333, although a lot of people seem to have received them, often in the form of two blank text messages. It may be a malfunctioning advertising number in which case it is nothing to worry about, though is understandably annoying.
When I worked for a mobile network, we were told to advise people to try and reply with the word “STOP” or “STOP ALL”, which would usually resolve the problem. Sometimes, however, you are unable to reply to the text message in question, in which case it may be worth contacting your airtime provider to see if they have any information about who it may be, or how to stop any further messages coming through in the future.
