Three Grand Masters 

The Three Grand Masters were:

    1) Solomon, King of Israel 

    2) Hiram, King of Tyre 

    3) Hiram Abif, AKA The Widows Son.

The Word could only be given during the presence of all of the Three Grand Masters.  Thus when one of them was killed the Word was Lost as it could not be given since one of the Three Grand Masters was missing.  This is said to have led to the use of the Substitute Word, which is now in use.