what to do for GS preliminary3

The importance of previous years papers cannot be underestimated.I have seen a general inertia-people hesitate to do them thinking whats the use they are unlikely to repeat in any case!Flawed thinking friends.you cant plan without knowing,you cant know without seeing and solving!
by their nature MCQ has four options.I got 70 right in Political science by merely going through the last years papers and following up on all four options completely.
Repeated perusal of previous years papers makes you reasonably clear as far as relative importance of various sub topics is concerned.That will help plan when you sit down to planning.
Take as many mock tests as you can,I dont necessarily mean those conducted by coaching classes,there are other options too.JTS institute conducted an all India free mock test last year online,and there are more such tests freely available online.In this case if you do prefer a coaching class like Vajiram there should be no harm done (if you dont let poor results affect your confidence)
solving repeated MCQs makes you more comfortable with the pattern of questions,gives a better idea of how much you  can guess safely in the mains exam.So this gives you both familiarity and a sense of confidence in the exam.

now,for the tricky part
1.for those with lots of time,there is a fat book by TMH on general studies for civil services,an alternative is available if you buy Pearson publication.This one book covers everything,and mastering this will see you safely through,but its not easy to master.
2.NCERT books are a safe bet if you can spare the time and read them from middle to high school.
3.TMH also comes out with section wise books-in history,geography and so on.If buying multiple books is all right,then these too are detailed and informative and well integrated with GS mains.
an alternative is similar books by spectrum or even wizard publications.

i must caution you,all books are good,provided you study.There is no magic pill and no magic book.UPSC will not use a single source to derive questions and if one is sufficiently well read any book they derive questions from,you will still answer.

lastly and most importantly-
Mental ability should be stressed on.Changes in pattern will be reflected increasingly in GS paper too.Mental ability,logic and reasoning are smallish in number of questions,but if you dont prepare for them and they are asked for more,bid your chances of passing bye bye.

personally,i will devote more time to Polity,Constitution, and the like,where the overlap with CSAT is more likely.
all the best!