I dislocated my shoulder almost 3 years ago why now do I still have bad pain there?

It also feels like it sometimes tryes
to dislocate. What shall I need to do?
During a dislocation of a shoulder many issues may occur. You might have torn
the rotator cuff which is allowing chronic subluxation (partial dislocation) of
the joint. You may have impinged a nerve, or you may have irritated and
inflammed the bursa causing chronic bursitis.

This shoulder, if it is causing the issues you are describing needs to be
examined by a physician, PA or Nurse Practitioner, but preferably at an
orthopedic clinic. It is likely they will order an MRI of the affected shoulder
to determine the problem.

Following the examination and diagnostic study, you might need an arthroscopic
repair, or you might just need rest and physical therapy.

In addition to working with orthopedic patients since 1978, I have had 16
shoulder dislocations myself (counting 9 on the right and 7 on the left). You
need to be evaluated.
Long term problems with joints is a very common occurance. Joints (ligaments
especially) are quite avascular - poorly circulated - and stretching and
scarring is comon. Once a shoulder dislocates, a reoccurance is also easily
possible (especially if you didn't have strengthing exercises and physical
therapy afterwards).

An ortho (=bone) pedic is a physician specialized in the muscle-skeletal system
of the human body. You should see a health care provider and watch that you do
not expose yourself to extreme forces on the previously injured side (a repeat
dislocation with aggravation of the injury is quite possible).
follow up with your orthopedist.