I think 3 feet high is a good height for roosts, not nest boxes. Our nest boxes vary from 1 foot off the ground to 20 inches.
Depends on your chickens and also whether you are hoping to have the hen hatch a clutch or you're just gathering up the eggs. Gathering eggs and your hens can fly that high? Tis okay. Lettin Mama hatch a clutch, the box needs to be waaaay lower.
~ if you do have a nest box that high, and really plump heavy breed hens, you might want to provide a "mid-way perch" so they don't stress their legs or damage their feet on landing.
My full-sized Buff Orp and Australorp have no problem jumping up a series of perches to get to their nest four feet off the ground.