In reply to dewsbury:
Depends on where you are in Nottingham but if you have easy access to the M1, then heading to the Derbyshire/Peak District is within the 1 hour (such a Dove Dale or Chatworth).
I've always found it difficult to find good walks near Nottingham and tend to head to Derbyshire!
Perhaps look at the map around:
Belvoir Castle.
River walks around Burton Joyce, Gunthorpe, Hoveringham.
Bestwood or Rufford parks.
Newstead Abbey (really interesting gardens).
Sherwood Pines.
Attenborugh nature reserve.
Some of the parks/reserves may be a bit short to fill 3 hours.
Drive just over the hour to get to Hope (e.g. Mam tor) or Derwent/Ladybower reservoirs for really fantastic walks.