What does it mean if someone sends you a message typing just simple letter '3'?

If someone sends you just the letter '3', it could be a playful or teasing way to convey affection, as it resembles a heart shape when turned sideways. Alternatively, it might be a shorthand for something specific between you two, or it could simply be a mistake or random keystroke. The context of your relationship and past conversations can help clarify its meaning.