Their website mentions that 3 SAT Subject tests are required, but I'm wondering if an IB diploma would work as an alternative.
Georgetown strongly recommends that you take three SAT Subject Tests to be considered for admission. These tests aren't explicitly required, but you should take them if you want to present the strongest application possible. The IB diploma and SAT Subject Tests are too unrelated for one to substitute for the other. While your IB qualifications may improve your application, they will not replace the Subject Tests. I'd definitely advise you to take three Subject Tests if Georgetown is one of your top choices.