These are the Hebrew letters (right to left) Bet, Samaech, and Daled. In English, this would be B"SD. It stands for "B'Siyata D'Shmaya", which is Aramaic for "With the help of Heaven". Many people put it at the top of documents, including Web pages, as a sign of respect for "You-Know-Who" (or would that be "Hu" ;-) , a way to ask for Divine Assistance in getting the document to it's source, and an acknowledgement of where the creative juices came from that made it possible to write the document.

For more information, ask a Rabbi, or see the following document from the [Ohr Someyach] [Ask the Rabbi] Archives, on the [Virtual Jerusalem] Web Server: