Did someone specifically tell you that you can pursue all three subjects to Honours level? Because I go to St Andrews, and I was under the impression that the most you could do was a Joint Honours degree with two subjects. I'm pretty sure you simply won't have enough time in your schedule to do all three the entire way.
That said, I'm doing Joint Honours Comp Sci and Econ and I will definitely be fully qualified in both by the end of my time here.