You definitely don’t have to wait for D3 schools to reach out to you; most have Prospective Athlete questionnaires that you can submit on their websites.
Just for example, since I’m familiar with the Claremont Colleges…
Claremont McKenna, Harvey Mudd, and Scripps share an athletic program: https://www.cmsathletics.org/sports/wten/recruitquestionnaire
and Pomona and Pitzer share another: http://www.sagehens.com/sports/wten/RecruitQuestionnaire
As for the Claremonts specifically, Claremont McKenna is a tippy-top Econ school; and foreign languages, which are shared among all the colleges, are top-notch. Pomona would be excellent for you academically but doesn’t offer merit. Scripps has good math/econ in its own right as well as ample cross-registration opportunities with the other schools, but CMC’s Econ department is the exception - those courses are generally not open to other Consortium students. (Mudd is a STEM-specialty LAC and wouldn’t be the right fit for you.) CMC, Pitzer, and Scripps all have merit scholarships - Scripps has the most (up to half-tuition) but CMC has some generous ones too. Pitzer, I believe, has only smaller merit awards unless there’s something I’m not aware of. Instrumental music wise, Pomona has its own ensembles, and the other four schools have a joint music program.
The academic level of D3 schools runs the gamut. There are some that rightfully wouldn’t be on your radar at all, and some that are too elite to offer any merit, but definitely quite a few in the target zone in between. (Scripps and CMC being two good examples, with wonderful tennis weather!)
There’s not much, if any overlap between marching band programs and D3 athletics, because typically having a band on the field is a D1 attribute. (Maybe other people can point out exceptions?)
But in the good-schools-with-a-serious-marching-band category, look at USC (good merit potential, and there’s club tennis if their D1 team is out of reach http://www.trojantennisclub.com/ ), Northwestern (I don’t think they give merit though?), and U of Miami (you could probably get significant merit there, and academically it wouldn’t be a bad school for your interests). All of these have conservatory-level music instruction and concert band opportunities as well, of course.
Another top school for you to look at would be U of Rochester. Phenomenal music (Eastman Conservatory is part of the University and there are ample opportunities for non-majors), D3 tennis (nice indoor facility, which is obviously needed because Rochester is crazy-cold), top-tier academics with an open curriculum that makes interdisciplinary/cross-disciplinary pursuits easy, and great merit aid potential. In addition to the expected math + econ offerings, this certificate might be of interest: http://www.rochester.edu/college/msc/certificates/mathmodel.html