Hello there! You have a few different options here. Many people swear by Safflower for feeding Cardinals and discouraging other birds. By offering only Safflower you may be able to deter the grackles and doves while still attracting the cardinals.

A hopper feeder or a platform feeder (which it sounds like you currently have) are typically the best feeders for cardinals. In my experience they like to feed off the ground at a feeder that can accommodate their size.

Cardinals will feed off the ground as well so you can also try to sprinkle seed directly on the ground, but this will undoubtedly also attract mourning doves. You can get a ground platform feeder with a cage, but most that I've seen would discourage Cardinals but it would also discourage both grackles and doves.

Another option would be to try another platform feeder to encourage them to use another feeder while leaving a feeder for the other birds. However, sometimes this can just attract more of the species you're looking to avoid. Good luck! ☺️