Is it possible to clear CAT'18 in 3 months being a final year student of CA?

Yes, 3 months of sincere effort is good enough to crack CAT. You should join coaching classes to get a better understanding of the kind of questions that are asked and also attempt a lot of mock tests.

The syllabus of CAT and CA is very much different. You just have to prepare like any other person preparing for CAT. And yes, it is possible to score good in CAT in 3 months.