"The heresy of heresies was common sense... "If both the past and the external world exist only in the mind, and if the mind itself in...

What this means -- what Winston is concluding -- is that
the Party needs people to believe whatever it says.  It needs them and commands them to
disregard what they know and to believe only what it tells
them.

The case of the three men you mention shows this.  It
is clear from the picture he finds that the men did not commit the crimes that they
confessed to.  But this shows Winston how much reality has changed.   He and others who
have his job had changed reality so much that no one would ever take this picture
seriously.

When Winston sees the picture, he realizes how
the rewriting of history makes true memory and use of common sense very difficult in
this society.  People have to just rely on what the Party says.