topic... I am travelling there but I new...
I read up on Mount Fuji.. enough time?
You need one full day to go Mount Fuji if you are staying Tokyo area. Usually going in the morning, about 4 hours from Tokyo one way by bus, come back late afternoon.
Ask your hotel to arrange tour bus going there, that's the easiest way. Of course can also go yourself with train, but rather troublesome if not familiar with the train system and where to change train, buying tickets, etc.
If you don't have 3 full days as in arrive one first day, leaving on third day, I suggest just walk around Tokyo area like Shinjuku, Shibuya, Harajuku, etc on the first and last day. Can also go to shrines if that interest you.
Wear comfortable shoes, will definitely do tons of walking.
Have fun!