is it necessary to feed our hens 3 grain scratch everyday outside of them getting their layer feed, they roam the yard all day,
getting the bugs and whatever else the find.
It in not necessary at all to feed scratch grains. It is considered a treat.
Most layer rations are around 16% protein so if you consider adding the grains to their feed be sure to check that the grain doesn't dilute the protein level (ie generally scratch grains are low in protein).
A better way is to limit how much scratch is given (about a tsp per bird), offer some fruit, veggies and greens.