Be very careful.
Likely a break in the line at a receptacle.
If you are lucky, it is at either the left most or right most receptacle.
If not, the break is at a receptacle to the left or right of those three.
Do look: is the hot still there but the neutral gone away?
Do you understand the last question?
Has someone recently installed a gfi without having the first clue as to what he was doing? ( a recent service call for me!)