My understanding is that some D3 tennis programs have tryouts and will take a few walk ons. The only way to know for sure is to contact the coaches. I have no idea about W&L - it’s possible. OP was also asking about places like MIT and Chicago which traditionally have been top D3 tennis programs that are unlikely to take walk ons. I kind of doubt any varsity teams are letting unranked players who aren’t interested in competing practice with them. Most schools should have intramural tennis if they don’t offer it as a club sport.