What are the three main themes of Twelve Angry Men?

I am not too sure why you feel that there are only three
main themes in this play. It is clear that any work of literature opens itself to a vast
number of different interpretations, and this excellent play is no exception. However,
the play does suggest in principal three general themes that we can identify as
follows:

1) The nature of
justice.

Clearly, the play's ending shows true justice
being done. What begins as apparently a clear cut case, with various jury members
wanting to get home to carry on with their lives, is saved by the stubborness and
persistence of Juror Eight. However, it is made evident that justice is only won after a
battle that suggests the fragile nature of justice. How many other jurys would have
quickly made the wrong decision to save themselves the problem of seriously debating the
case?

2) The responsibility of
justice.

A part of the play that it is important not to
ignore is the introduction that the Judge gives to the jurors, informing them of their
role and responsibility:

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I urge you to deliberate honestly and
thoughtfully. You are faced with a grave
responsibility.

The play
points towards the vital role that we all have in ensuring that true justice is enacted.
The play is as much a comment upon the justice system and our social responsibility as
it is an excellent, gripping drama. Concepts such as justice, which the USA is based
around, are only as strong as the individuals that help support and affirm such
concepts.

3) The blindness of
justice.

Traditionally, justice is depicted as a blind
woman, carrying a sword in one hand and scales in the other. This blindness is essential
to ensure that justice is not swayed by color, creed, or religion. Of course, this is
something that Juror Ten does not exhibit. Note how he characterizes the defendant whose
fate is being decided:

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Bright! He's a common, ignorant slob. He don't
even speak good
English!

Prejudice is
something that represents a barrier to true justice being enacted, and this is another
central theme of the play.