Hello Cathy,

yes they can bring them back to 3 feet , no problem.
do that when it's not to hot , it's better for the plants .
they will re -grow, but the more you cut-off , the longer it will take . it's better not to cut in the very old wood .
if my english was better , i could explain it better.
all the best ,

Wim , the boxwood collector

I am facing this problem myself. My own fault, I'm afraid; I've lived here 33 years and basically ignored them all that time. I was afraid that if I started to shear them immediately I'd be dealing with hollow interiors, so I chose to start trying to first thicken them up by removing selective branches from WITHIN, to let in light and air. After doing this off and on for two years I feel ready to do some major cutting back now; I am optimistic that it will work.

Here's a pruning article

Here is a link that might be useful: link