3-point sliding window

first input:

             x    x    x    x    x    x    x    ...
             |
             V
y1    y2    y3

second input:

             x    x    x    x    x    x    x    ...
             |    |
             V    |
y1    y2    y3    |
       |     |    |
       V     V    V
      y1    y2   y3

third input:

             x    x    x    x    x    x    x    ...
             |    |    |
             V    V    |
       y1   y2   y3    |
             |    |    |
             V    V    V
            y1   y2   y3

fourth input:

             x    x    x    x    x    x    x    ...
                  |    |    |
                  V    V    |
            y1   y2   y3    |
                  |    |    |
                  V    V    V
                 y1   y2   y3

In general:

	y1 = y2;  y2 = y3;  y3 = next input value