First of all, weigh your flakes, that's all I can say. I've been told by every hay seller so far that bales weigh 110lbs but have yet to see a bale which actually does....maybe the ones they keep.
How do I know? I weigh every serving for my horses. And a 4-5" flake if alfalfa weighs about 7 lbs, 3-way even less.
Then it depends on the quality, especially of the 3way. Could be all straw, when baled late=very little nutrition.
Do they clean up the hay completely, also the 3way? If yes, they need more. Period. Or something extra. If you give them some sort of feed one week out of the month for the psyllium, why not the whole time? That might be just what they need extra. Could be alfalfa pellets with some oil or other fat source, and a vitamin/mineral supplement. The blocks DO NOT PROVIDE SUFFICIENT, are hard to get enough off. They're meant for cattle.
Try to get some pics up
Another way to see if they're underweight : Google the Henneke Body Condition Score, print out the instructions and score every horse. If they're below 4,5 they're too skinny. That your dad might just understand....it's being used by vets. If they all come out at around 5, yo have nothing to worry about