When my D completed the classes required for her math major at a selective LAC, she decided to add a double in CS which has kept her quite busy. I don’t know off-hand the exact 300 level math classes but I believe they included graph theory, dynamical systems, and abstract algebra to name a couple. She was first interested in the Budapest Semesters in Math program but ultimately decided and was accepted to the Budapest AIT for Computer Science and Software Engineering program because she didn’t want to get behind in her CS mandatory classes. I only say this to let you know that it is possible to double in Math and CS at other than a highly selective school or top 20 LAC, at which not many are accepted and others might not be able to afford.