interchangeable hubs?

I have an old huffy 3-speed, my best guess is that it is from the
1970's. I'm just learning how to maintain and repair my own bike, and
was wondering if it is possible to convert the current Shimano 3-speed
hub to one of their new Nexus 7-speed hubs. Where would I start to
figure out if it's even possible? (Tire size, frame fork, something
else?)