Applying for a PhD with two undergrad degrees in different subjects
The way I see it (but I am not in any admissions committee), the career choice that you did being 18 should have very little impact now. At that age, most people are immature.

For me, having those two degrees tells me:


You have had the persistence to finish the Philosophy degree, even knowing that that was not your cup of tea; and then pursue your real desire.
You have the flexibility to work on very different intellectual environments.
Your experience is broader than most people that start their Maths degree at 18. Also, you are in Mathematics after a concious decision, and it was revalidated when you pursued a MSc, and now applying for a PhD.


In any case, I think you should absolutely mention it, otherwise you would have a 3 years gap to explain. Having been studying is definitely better than doing nothing.