This question is quite general, so is difficult to answer. For ornamentals, look at taller woody plants such as evergreens along with deciduous plants that have interesting bark and branches such as red-twigged dogwood/Cornus sericea or colorful berries such as winterberry holly/Ilex verticillata or Siberian crabapple/Malus baccata 'Halward' which is 15' rather than the large sized tree of the species. Here's a link to a list of ornamental plants with winter interest that are hardy to zone 3 from the Missouri Botanic Garden's plant finder. You can click on any of them to get a whole page with details on the plant. If you want to do your own search, you can leave blank everything but your zone or you can fill in just some of the other factors, such as evergreen and tree or shrub and fruit

For edibles, you can extend your harvest (see Eliot Coleman's books for info on winter harvesting from a cold frame or unheated high tunnel) but probably won't be able to harvest all winter, though we eat veggies we've grown and stored in a cold cellar through April most years.

What you can grow in your zone also depends where you live... That leaves places in the West, MidWest, and Northeast. Here's just a quick list of general plants hardy to Zone 3: Blue Delphinium, Bunch berry, Creeping Wintergreen. There are tons more ideas here- http://www.waysidegardens.com/cold-hardy/c/WG_Cold_Hardy/ Use this link along with NHBabs

You might also do some reading on the far north forum.

Zone 3 is just "how cold does it get" and pretty useless for much beyond knowing what you are unlikely to kill. That Zone system has Phoenix AZ and Tampa Florida in the same zone!
Where - City and state - do you live?