The choice is always upto you, if you want to pursue maths or botany through your career, the opportunities might vary as you might be refering to the triple main course. As PCB student, it would be difficult for you to apply in the mathematics courses as they would require your marks in mathematics as criteria for admission. If that is not the case unlike PCB courses directing you towards pharmaseutical, biotechnology, nutrition and biomedical field as  varient career options, Physics, Chemistry, Maths will redirect you towards technical fields like Electronics.

As you have mentioned, you have a base in Physics, Chemistry and Biology, it would be easier for you to build a career based on your strong points as you will have to deal with mathematics anyway as a sub paper in physics and chemistry rather than taking it as a major. Botany can ample opportunities for you as well. After all it all comes down to your interest.

I wish this was helpful. Best wishes.