Ups is not obligated to make you serve the suspension if they choose not to. If you grieved it and lost, you would be still subject to progressive discipline and therefore get the the 3 day suspension.
My thoughts are;
1. If your attendance policy, like where I am, is 5 occurrences in a 9 month rolling window, then either you just took your 6th occurrence or their calculations are wrong.
2. If you learned your lesson from not having a note from your Dr the first time, you should have known this time.
3. The next step is out the door. You need to learn and understand the policy where you are and abide by it.