Define the system as the 6 hydrogen atoms. Its initial rest energy consists of the rest energy of 6 hydrogen atoms.
After the fusion reaction, there is one atom of helium and two hydrogen atoms. Their rest energies are the final rest energy of the system.
The change in rest energy of the system is
(Note: all of the figures in the answer are significant.) It makes sense that rest energy is lost because this loss of rest energy results in gains of other types of energy, according to the Energy Principle. Where does the energy radiated from Sun come from? Ultimately, it comes from the loss of rest energy during the fusion reactions that occur within Sun.