One way is to use your eyes as much as your ears. If you can see that one string isn’t vibrating or is vibrating much more than the others, that means you need to put in some more work. It helps to place the three notes and then practice playing each finger individually, too. Don’t always do this in the same pattern, like 321, though. Do 231, 132, 312, etc, as well.
Thank you very much – I’m trying that right now, and yes, I must say I am amazed at how inconsistent the volume of the notes is when I play them in a different order! DEFINATELY something to work on there!