Pursuing BSc maths after taking science in your 11 and 12 is the only option that you have. You don't have to go through any kind of entrance exams for getting bsc mathematics just you need the HSC boards exam, and a good percentage in them will ensure your seat in a good college as well.

And after completing BSC in mathematics and getting the degree, you can pursue courses like MCA, M.Sc IT, actuarial sciences, MBA or M.Sc in Mathematics .

Refer to the links below for more detailed information..

https://www.careers360.com/colleges/government-college-madappally/bsc-mathematics-course

Thank you.

If you are not interested in degree course the you can go for B Tech since its a engineering course s it will be an advantage for job also,  but there is difference between B Sc and B tech , instead of 3 years B tech will take your 4 years so choose accordingly