Topics are diagram based hence they are combined together. Maximum weightage is given to geometry among the three of them in QA , although every CAT paper will have 3-4 questions on mensuration, as well as a couple of questions on coordinate geometry.
Topics that need to be covered in geometry are basic theorems involving triangles, circles, parallel lines and other shapes. Similarity and Congruency of triangles is an important property to look after for as they shape the pattern and type of the questions.
In coordinate geometry of QA section, straight lines and circles hold their due importance. Given the equation of a circle, you should be able to comprehend the centre and radius of the circle and plot it with respective intercepts.
Tangents and other lines can add up to the complexity. Similarly, you should know what the slope and y-intercept of a given straight line equation is, and be able to draw the line on a piece of graph paper. This will help in forming its equation and solving the problem further.