Three of something, like whistle blasts or rifle shots, often means danger. SOS is three dots three dashes three dots so it's three threes. It's also handy that it can mean save our ship. The signal was introduced as a radio telegraph distress call in the early 20th century, replacing another signal which I've forgotten. I believe the sinking of the Titanic was one of the first uses of the new SOS distress signal.