Including incomplete or complete BS after MS in résumé
I think the most important question is what are you using this resume for? You say you're "about to start on a PhD program also in Math" -- does that mean you already got accepted? Are you looking to get summer jobs?

My main concern is if you have a PhD degree, a Masters degree, and another Bachelors degree, you won't have enough space to list other degrees (especially if you want to list relevant coursework). You'll hopefully get teaching experience, work experience, and leadership skills to fill out the rest of your resume.

If you don't have another Bachelors degree, what did you list when you were applying to the Masters? I would just follow whatever you did there. It might seem odd that you have a Masters but no Bachelors degree.