Michigan is only requiring them all day for 6-12. Kids in K-5 are to wear them in hallways and common spaces and when moving around, but not when at their own desks and appropriately distanced. I wish our middle school had the same flexibility - DD11 only has four kids in her class for fall, but even with being able to space more than 6', they'll have to be masked all day long.
I think the three feet is a nod to necessity. There's no way for most schools to accommodate 6', even with A/B shifts, but 3' might be doable. Of course, there are bound to be some that can't manage any additional distance at all and I really wish they were offering more guidance for how to handle those situations, particularly since many of those places also have the greatest challenges to distance learning and are therefore reluctant to go the online/hybrid route.