All 3 schools will provide a high quality education. I went to medical school with graduates of all 3, and all were well prepared.
Gustavus attracts mainly students from MN with a few from neighboring states, not a lot from elsewhere in the country. It has a good regional reputation.
St. Olaf is increasingly gaining national attention. Starting to attract some kids from California etc. A College That Changes Lives (CTCL) school. Standout departments are Biology, Math and Music. Just announced a new president. Allows cross registration at Carleton. Gorgeous campus. Great food.
Macalester has a bigger national reputation than either of the other 2, attracting some kids from both coasts and internationally. Located in a fun urban neighborhood. Quirky vibe, very liberal. Standout departments include Poly Sci, with lots of internship opportunities at the nearby state capitol.