As mentioned earlier GS -2 and GS -3 are totally dynamic papers.GS- 3 paper is generally on expected lines. If the student has studied the topics properly questions can be easily predicted.
GS-3 broadly covers 5 topics

i) Science & Technology
ii) Indian Economy
iii) Ecology, Biodiversity and Environment
iv) Internal Security of India
v) Disaster prepredness and management

Science and technology is a very scoring area.Questions usually is easy and predictable. To study basic sciences again NCERT’s are the best resources, which the candidate has already covered during prelims. A quick revision of NCERT’s from class VI to X is required. The other topics i.e. development in the field of science & technology is quiet detailed. Spectrums book on science and technology is sufficient to cover this topic. It gives you a basic idea of what is to be covered. The candidate should religiously follow the science and technology page every Thursday in the Hindu. Surely questions from topics like Biotechnology Nanotechnology, Robotics and Indian defence are asked. The questions are from latest developments that have taken place in this field. Internet is a constant help to prepare science and technology.

To prepare economy first of all the basic economic concepts should be in place any basic book of Economy which is referred during prelims i.e. either Ramesh Singh or Access Publication or Sanjeev Verma is useful.

One should also follow the five year plan document and the economic survey to prepare topics such as Growth. Economic survey is the most reliable and effective sources to prepare multiple topics in economy. Budget is another government document which is mightily effective to prepare economy for mains. All of government budgeting questions are directly asked from the Union Budget