Just pulling the definition of T3A from the Phoenix5 web site:

Stage T3a disease

 All forms of stage T3 disease require extension of prostate cancer
tissue through the so-called "prostatic capsule" and out of the prostate
into the immediately surrounding tissue. In the case of stage T3a
disease the prostate cancer tissue must have extended outside the
prostate capsule on one or other side of the prostate only (unilateral
extracapsular extension). Thus stage T3a is very similar to some forms
of stage C1 in the Jewett-Whitmore system.

If you have Dr. Walsh's book, he discusses extension through the capsule.

I'm not a doctor, but my understanding is that in that case, the cancer
has not truly escaped and gone anywhere.  But you may need local
radiation at the site just to be sure.

jimhoney

What did the path report say about positive margins?  It's entirely
possible to have extension as you have and yet cut out enough of the
surrounding tissue to get all of the cancer in which case you're no
worse off than anyone else.  The problem occurs when the surgeon slices
through this extension leaving a piece behind.  This would be reported
as a positive margin.
Dave Perry

According to Walsh's Guide to Surviving Prostate Cancer, if the margins
are clear and the pathological Gleason score is 6, then your chances of
avoiding biochemical recurrence within ten years is about 95 percent.
So you shouldn't be too worried at this stage if the margins are clear.
 If there were positive margins,  the situation is still not too bad.
According to Walsh the likelihood of avoiding biochemical recurrence
within ten years is at least 72 percent.   But you should read Walsh
yourself on the subject.  You can also go to the Sloan Kettering website
which has a calculator you can use to estimate the likelihood of staying
PSA free.  Finally, you should discuss this all with your doctor.