If one of three beneficiaries has died, what happens to their share of the estate?

I am so sorry for your loss.  Thatn you for qutoing the line after but the Last Will and testament needs to be read in its entirety.  There should be a line or paragraph that says something about pre-deceasing the testator (the person who made the Will), which is what I am assuming has happened here. The Will should state that either that parties share goes to the other beneficiaries of to the decendeants of that beneficiary.  Now, if the beneficiary dies after the Testator his or her portion passes through his or her own estate.  Good luck.