The order would be: Psychology...Zoology...Indo-Pak History.
Reason: In CSS, we people have to keep in mind the syllabus, trend, our own interest respectively. So, when we come to syllabus, Psychology has comparatively less syllabus than the other two. When we talk about trend, psychology is at first place. Last but not least, your interest and educational background puts you to study either Psychology or Zoology but not Indo-Pak history.
Next stage is what to opt between the two. Being a medical student, you can safely, easily and excellently cover Psychology. It can surely give you the chance to fetch more marks than Zoology. People who have no medical/bio background opt Psychology, then why don't you? In the end, you'll hanker after your liking subject but after fetching more scores. The need can be fulfilled by opting Psychology esp. when you are Medical Student. dear fellow!

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"But screw your courage to the sticking place,
And we'll not fail." _Shakespeare, 'Macbeth')