There are a number of factors that determine if an intern will be hired as a full-time employee, only some of which are under your control.
It's possible that the company does not hire interns, or hires only a small percentage. It could be that there are different standards and requirements for a full-time employee than there are for interns.
This is all to say: don't worry about what you cannot control. Your best chance to be hired as an full-time employee is to act like one. Don't sell yourself short because you're "only an intern."
Do your best work over the 12 weeks. Ask questions if you're stuck or unsure what to do. Be proactive. If you don't have work, find out what you can do to help people. Look for systems, processes, or tasks you can improve, increase efficiency, or eliminate waste. Make sure you get permission before making changes.
Take notes. Pay attention to everything that's going on around you, even if it doesn't seem relevant at the time. Be someone who people can count on.