Since you are studying B.Sc with Physics, Mathematics and Geology and are interested to have a post graduation degree in any one of the three for your better career prospects. In this connection, I would suggest you to choose the subject for which you have sincere interest and passion. This will help you in understanding the complexities of the subjects.
Now, take the case of Mathematics, it is itself a very vast subjects dealing with different papers such as Differential - calculus, Differential - Equation, Integral - calculus, Vector, Hydrostatics, Electricity and Magnetism, Linear - algebra, Coordinate - geometry, Solid Co- ordinate Geometry etc. So without your interest, the subject matters would appear to be boring for you. In case, you show your exceptional performance in this field, you are sure to achieve a respectable post in the different fields like teaching, Research, Central - government jobs as a scientist.
If interested for Physics, it is really a interesting subject and will stimulate you by going through the different topics such as Nuclear - physics, Thermodynamics, Electronics, Quantum - mechanics etc and in almost all the branches of Physics, you need to have clear understanding of Mathematics since mathematical - calculations are involved at each stage. A good score in the Post - graduation level will open your avenues in Reseach, teaching in colleges, scientist in ISRO etc.
Geologists are in good demand in Mining and Petrolium - field apart from teaching profession.
In all the subjects as mentioned above, aquistion of a Doctorate - degree will multiply your career - prospects in your chosen field.