Welcome to Simply.
Your tank is way too large for 3 3 1/2" Discus, and 3 is not a good number of fish. You should have at least 5. 6 would be better.
I always drop and plop. They appreciate the good clean water way more than they care about the different water parameters. Most all of of us here drop and plop. I would raise the temp to 29. Let us know how they are in the morning.