If you are working on other Crimson Dragon medals, achievements and challenges this will probably come naturally. However, if you're going for this challenge solely by itself, I will list my favorite method. Go to Coral Lakes East and select the Bloated Anomaly mission. Be sure to bring a wingman with you. You can get three S ranks in just a few minutes.
Please note, the challenge achievement counts each mission section rank, not just the final rank for the mission. So even if you earn S A and S in a mission, you'll still get credit for the two S's you earned.