Before feeding feed/grains please make sure their dental work is current.
Horses just eating free-choice pasture should not
be thin if adequate pasture grass was in abundance.
A sharp tooth or dental issue could be contributing to the less than optimum look you are seeing.
Being all 3 horses are on the thin side I would make sure that first their medical needs were done, teeth and worming for sure.
Once that was done and the horses are provided with ample supply of nutritious hay free-choice... only then would I add a grain/feed supplement to their diet.
The fact you write they are loving the mineral block makes me think they may not have had all their needs looked after...
3 horses, various ages, and all thin to some degree...red flag
that they need someone to check out further their "care" needs...updated as necessary. They just came out of the best grazing time of the year with nutrient rich grasses....
Best of luck with the new herd.